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Algebra

Algebra Help: Deriving Applying The Geometric Series Formula

Review real example questions for Deriving Applying The Geometric Series Formula in Algebra.

Question 1

Derive the finite geometric series formula starting from $S_n = a + ar + ar^2 + \cdots + ar^{n-1},$ by multiplying by $r$ , subtracting to cancel middle terms, and solving for $S_n$ (assume $r \ne 1$ ). Which final expression is correct?

$S_n = \frac{a(1-r^n)}{1-r}$
$S_n = a + (n-1)r$
$S_n = \frac{a(1-r^{n-1})}{1-r}$
$S_n = \frac{a(1-r^n)}{1+r}$

Explanation: This question tests your understanding of geometric series—the sum of terms from a geometric sequence—and how to use the formula

S_n = a(1 - r^n)/(1 - r)

to calculate these sums efficiently. The geometric series formula

S_n = a(1 - r^n)/(1 - r)

comes from a clever trick: write the sum

S_n = a + ar + ar^2 + \cdots + ar^{n-1}

, then multiply by

r

to get

rS_n = ar + ar^2 + ar^3 + \cdots + ar^n

. Subtracting these (

S_n - rS_n

) makes almost all terms cancel, leaving just

S_n(1 - r) = a(1 - r^n)

, so

S_n = a(1 - r^n)/(1 - r)

. The middle terms canceling is the magic that makes this work! Deriving

S_n = a(1 - r^n)/(1 - r)

: (1) Write the sum:

S_n = a + ar + ar^2 + \cdots + ar^{n-1}

. (2) Multiply both sides by

r

rS_n = ar + ar^2 + ar^3 + \cdots + ar^n

. (3) Subtract second from first:

S_n - rS_n = (a + ar + ar^2 + \cdots + ar^{n-1}) - (ar + ar^2 + ar^3 + \cdots + ar^n)

. (4) Notice the cancellation: all middle terms cancel, leaving

S_n - rS_n = a - ar^n

. (5) Factor left side:

S_n(1 - r) = a(1 - r^n)

. (6) Divide by

(1 - r)

S_n = a(1 - r^n)/(1 - r)

. This derivation shows why the formula works—it's not just memorization! Choice B correctly derives the formula through the subtraction method with the accurate final expression. Choice A makes an error in the derivation by using

r^{n-1}

instead of

r^n

in the numerator; this happens if you miss that the remaining term after cancellation is

-ar^n

from the shifted series. Derivation memory aid: the trick is writing

S_n

, then writing

rS_n

(shifted one term), then subtracting. When you subtract, the middle terms align and cancel:

S_n

has 'ar' and

rS_n

has 'ar' (opposite signs, cancel!),

S_n

has 'ar^2

and

rS_n

has 'ar^2' (cancel!), etc. Only a from

S_n

and

-ar^n

from

rS_n$ don't cancel. This telescoping is the insight! Once you see it, you'll never forget the derivation.

Question 2

For the geometric series $2 + 6 + 18 + 54$ , identify $a$ , $r$ , and $n$ , then find the sum using $S_n = \frac{a(1-r^n)}{1-r}$ .

$a=2, r=2, n=4; S_4=30$
$a=2, r=3, n=4; S_4=80$
$a=2, r=3, n=3; S_3=26$
$a=6, r=3, n=4; S_4=240$

Explanation: This question tests your understanding of geometric series—the sum of terms from a geometric sequence—and how to use the formula

S_n = \frac{a(1 - r^n)}{1 - r}

to calculate these sums efficiently. A geometric series is the sum of terms from a geometric sequence: if the sequence is 2, 6, 18, 54, ... (multiply by 3 each time), the series is 2 + 6 + 18 + 54 + ... (adding those terms up). Rather than adding manually (tedious for many terms!), we use the formula

S_n = \frac{a(1 - r^n)}{1 - r}

, where a is the first term, r is the common ratio, and n is how many terms we're summing. This formula works for any finite geometric series! Let's verify the formula works for a simple example: series 2 + 6 + 18 + 54 with a = 2, r = 3, n = 4. Formula:

S_4 = 2(1 - 3^4)/(1 - 3) = 2(1 - 81)/(-2) = 2(-80)/(-2) = 80

. Manual addition: 2 + 6 + 18 + 54 = 80 ✓. The formula gives the same answer as adding manually, but it's much faster for large n! Choice A correctly identifies a=2, r=3, n=4 and calculates the sum as 80 with accurate arithmetic. Choice B identifies the parameters wrong: n=3 instead of n=4; getting a, r, or n wrong throws off the entire calculation! Why this formula is powerful: to sum 2 + 6 + 18 + 54 + ... + (2·3^99), you'd need to add 100 terms manually (impossible!). With the formula:

S_100 = 2(1 - 3^{100})/(1 - 3)

and you're done (calculator handles 3^100). The formula turns a hundred-operation problem into a few operations. That's the beauty of having a formula!

Question 3

What is the sum of the first 5 terms of the geometric series $2+6+18+54+162?$ (Identify $a=2$ , $r=3$ , $n=5$ .)

$242$
$80$
$121$
$244$

Explanation: This question tests your understanding of geometric series—the sum of terms from a geometric sequence—and how to use the formula S_n = a(1 - r^n)/(1 - r) to calculate these sums efficiently. A geometric series is the sum of terms from a geometric sequence: if the sequence is 2, 6, 18, 54, ... (multiply by 3 each time), the series is 2 + 6 + 18 + 54 + ... (adding those terms up). Rather than adding manually (tedious for many terms!), we use the formula S_n = a(1 - r^n)/(1 - r), where a is the first term, r is the common ratio, and n is how many terms we're summing. This formula works for any finite geometric series! For the series 2 + 6 + 18 + 54 + 162 with a = 2, r = 3, n = 5: Using the formula S_n = a(1 - r^n)/(1 - r), we substitute: S_5 = 2(1 - 3^5)/(1 - 3) = 2(1 - 243)/(-2) = 2(-242)/(-2) = 2(121) = 242. So the sum of the first 5 terms is 242. Choice A correctly applies the formula with a = 2, r = 3, n = 5 and calculates the sum as 242 with accurate arithmetic. Choice C gives 121, which is exactly half of the correct answer 242. This suggests forgetting to multiply by a = 2 in the final step: (1 - 3^5)/(1 - 3) = (-242)/(-2) = 121, but we need 2 × 121 = 242. Don't forget the first term multiplier a in the formula! Let's verify the formula works for a simple example: series 2 + 6 + 18 + 54 with a = 2, r = 3, n = 4. Formula: S_4 = 2(1 - 3^4)/(1 - 3) = 2(1 - 81)/(-2) = 2(-80)/(-2) = 80. Manual addition: 2 + 6 + 18 + 54 = 80 ✓. The formula gives the same answer as adding manually, but it's much faster for large n!

Question 4

A bouncing ball rises to $80\%$ of its previous height after each bounce. If the first bounce reaches $10$ ft, what is the total of the bounce heights for the first $5$ bounces?

Sum the geometric series $10+10(0.8)+10(0.8)^2+10(0.8)^3+10(0.8)^4$ (so $a=10$ , $r=0.8$ , $n=5$ ).

$\dfrac{10(1-0.8^5)}{1-0.8}=33.616$
$\dfrac{10(1-0.8^4)}{1-0.8}=29.52$
$\dfrac{10(1-0.8^5)}{1-0.8}=16.808$
$10(0.8)^5=3.2768$

Explanation: This question tests your understanding of geometric series—the sum of terms from a geometric sequence—and how to use the formula S_n = a(1 - r^n)/(1 - r) to calculate these sums efficiently. Geometric series appear in finance: when calculating mortgage payments, the total amount paid is a geometric series with the interest rate as the ratio. Each payment accumulates interest, creating the geometric pattern. The formula lets you calculate loan payoffs, investment growth with regular contributions, and other real-world money situations where compounding occurs! In the context of a bouncing ball, each bounce height is 80% of the previous: 10 ft, then 10(0.8) = 8 ft, then 10(0.8)² = 6.4 ft, creating a geometric series. With a = 10, r = 0.8, n = 5, the total is S_5 = 10(1 - (0.8)^5)/(1 - 0.8) = 10(1 - 0.32768)/(0.2) = 10(0.67232)/(0.2) = 10(3.3616) = 33.616. This shows the total distance traveled upward in 5 bounces. Choice A correctly applies the formula with a = 10, r = 0.8, n = 5 and calculates the sum as 33.616 with accurate arithmetic. Choice C has a calculation error: it shows 16.808 which is exactly half of 33.616, suggesting they might have divided by 0.2 incorrectly or made an arithmetic mistake. When 0 < r < 1, both (1 - r^n) and (1 - r) are positive, so the sum should be positive and match our calculation. Why this formula is powerful: to sum 2 + 6 + 18 + 54 + ... + (2·3^99), you'd need to add 100 terms manually (impossible!). With the formula: S_100 = 2(1 - 3^100)/(1 - 3) and you're done (calculator handles 3^100). The formula turns a hundred-operation problem into a few operations. That's the beauty of having a formula!

Question 5

Use the geometric series formula $S_n=\dfrac{a(1-r^n)}{1-r}$ to calculate the sum $3+3(1.1)+3(1.1)^2+\cdots+3(1.1)^9.$ (So $a=3$ , $r=1.1$ , $n=10$ .)

$\dfrac{3\left(1-1.1^{9}\right)}{1-1.1}$
$\dfrac{3\left(1-10\cdot 1.1\right)}{1-1.1}$
$\dfrac{3\left(1-1.1^{10}\right)}{1-1.1}$
$\dfrac{3\left(1-1.1^{10}\right)}{1.1}$

Explanation: This question tests your understanding of geometric series—the sum of terms from a geometric sequence—and how to use the formula S_n = a(1 - r^n)/(1 - r) to calculate these sums efficiently. To use the formula: (1) identify the first term a (what's the first number being added?), (2) find the common ratio r (divide any term by the previous term), (3) count how many terms n you're summing, (4) substitute into S_n = a(1 - r^n)/(1 - r), (5) calculate carefully. Example: for 5 + 10 + 20 + 40 + 80, we have a = 5, r = 2, n = 5, so S_5 = 5(1 - 2^5)/(1 - 2) = 5(1 - 32)/(-1) = 5(-31)/(-1) = 155. For the series 3 + 3(1.1) + 3(1.1)² + ... + 3(1.1)⁹ with a = 3, r = 1.1, n = 10: Using the formula S_n = a(1 - r^n)/(1 - r), we substitute: S_10 = 3(1 - 1.1^10)/(1 - 1.1). This matches the expression in choice A exactly. So the sum is expressed as 3(1 - 1.1^10)/(1 - 1.1). Choice A correctly applies the formula with a = 3, r = 1.1, n = 10 with accurate substitution. Choice B has n = 9 instead of n = 10: when counting from 3(1.1)⁰ to 3(1.1)⁹, that's 10 terms total (including the 0th power). Remember that powers go from 0 to n-1, giving n terms total. Off-by-one errors in counting terms are common! Common pitfall: confusing n (number of terms) with the last exponent. In a + ar + ar² + ... + ar^(n-1), there are n terms, but the last exponent is (n-1)! Count carefully: if the series is 'first 5 terms,' then n = 5, but the last term is ar^4, not ar^5. Off-by-one errors in n throw off the sum. The formula already accounts for this with r^n, not r^(n-1).

Question 6

A ball’s rebound heights form a geometric sequence: after the first bounce it rises to $10$ ft, and each bounce reaches $80\%$ of the previous height. What is the sum of the heights of the first 5 bounces (in feet)?

This is the geometric series $10+10(0.8)+10(0.8)^2+10(0.8)^3+10(0.8)^4$ with $a=10$ , $r=0.8$ , $n=5$ .

$\approx 33.62$
$\approx 40.00$
$\approx 26.89$
$\approx 36.16$

Explanation: This question tests your understanding of geometric series—the sum of terms from a geometric sequence—and how to use the formula S_n = a(1 - r^n)/(1 - r) to calculate these sums efficiently. Geometric series appear in finance: when calculating mortgage payments, the total amount paid is a geometric series with the interest rate as the ratio. Each payment accumulates interest, creating the geometric pattern. The formula lets you calculate loan payoffs, investment growth with regular contributions, and other real-world money situations where compounding occurs! In the context of ball rebound heights, each height is 80% of the previous, creating a geometric series: 10 + 10(0.8) + 10(0.8)^2 + 10(0.8)^3 + 10(0.8)^4. With a = 10, r = 0.8, n = 5, the total is S_5 = 10(1 - 0.8^5)/(1 - 0.8) ≈ 10(1 - 0.32768)/0.2 ≈ 10(0.67232)/0.2 ≈ 10(3.3616) ≈ 33.62. This shows the total height summed over the first 5 bounces. Choice A correctly applies the formula with a = 10, r = 0.8, n = 5 and calculates the sum as ≈33.62 with accurate arithmetic. Choice D uses the arithmetic series formula instead of the geometric series formula: arithmetic series use S_n = n(a + last term)/2 (for constant difference), but this is a geometric series with constant ratio, needing S_n = a(1 - r^n)/(1 - r). Don't confuse the two types! For financial applications like mortgages: the payment P, interest rate r (per period), and number of periods n plug into formulas built from geometric series. You might see S = P((1+r)^n - 1)/r or similar—these come from rearranging the geometric series formula! Understanding the underlying geometric series helps you understand why mortgage formulas look the way they do.

Question 7

In a savings plan, you deposit $\$ 100 $at the end of each month. The account earns$ 0.5% $interest per month. After 12 months, the value of the deposits is $$100\big(1.005^{11}+1.005^{10}+\cdots+1.005^1+1.005^0\big).$$ Use$ S_n=\dfrac{a(1-r^n)}{1-r}$ to find the total value after 12 months (in dollars, to the nearest cent).

$\$ 1{,}200.00$
$\$ 1{,}236.94$
$\$ 1{,}260.00$
$\$ 1{,}230.79$

100 grows by 0.5% per month, creating a geometric series: 100(1.005)^11 + 100(1.005)^10 + ... + 100(1.005)^0. With a = 100, r = 1.005, n = 12, the total is S_12 = 100(1 - 1.005^12)/(1 - 1.005) = 100(1 - 1.061677812)/(−0.005) = 100(−0.061677812)/(−0.005) = 100(12.3355624) = 1233.56. Wait, let me recalculate: the series goes from 1.005^11 down to 1.005^0, which we can rewrite as 100(1.005^0 + 1.005^1 + ... + 1.005^11). Using a = 100, r = 1.005, n = 12: S_12 = 100(1.005^12 - 1)/(1.005 - 1) = 100(1.061677812 - 1)/0.005 = 100(0.061677812)/0.005 = 100(12.3355624) = 1236.94. This shows the account value after 12 deposits is

1,236.94. Choice C correctly calculates the sum as

1,236.94 with accurate arithmetic and proper handling of the compound interest. Choice B gives

1,230.79, which might result from using 11 terms instead of 12, or from a calculation error. Remember that deposits from month 1 to month 12 means 12 deposits total, each earning different amounts of interest based on how long they've been in the account. For financial applications like mortgages: the payment P, interest rate r (per period), and number of periods n plug into formulas built from geometric series. You might see S = P((1+r)^n - 1)/r or similar—these come from rearranging the geometric series formula! Understanding the underlying geometric series helps you understand why mortgage formulas look the way they do.

Question 8

Use $S_n=\dfrac{a(1-r^n)}{1-r}$ to find the sum of the geometric series $7+7(-0.5)+7(-0.5)^2+\cdots+7(-0.5)^7.$ (So $a=7$ , $r=-0.5$ , $n=8$ .)

$\dfrac{120}{16}$
$\dfrac{1799}{128}$
$\dfrac{119}{16}$
$\dfrac{63}{8}$

Explanation: This question tests your understanding of geometric series—the sum of terms from a geometric sequence—and how to use the formula S_n = a(1 - r^n)/(1 - r) to calculate these sums efficiently. A geometric series is the sum of terms from a geometric sequence: if the sequence is 2, 6, 18, 54, ... (multiply by 3 each time), the series is 2 + 6 + 18 + 54 + ... (adding those terms up). Rather than adding manually (tedious for many terms!), we use the formula S_n = a(1 - r^n)/(1 - r), where a is the first term, r is the common ratio, and n is how many terms we're summing. This formula works for any finite geometric series! For the series 7 + 7(-0.5) + 7(-0.5)^2 + ... + 7(-0.5)^7 with a = 7, r = -0.5, n = 8: Using the formula S_n = a(1 - r^n)/(1 - r), we substitute: S_8 = 7(1 - (-0.5)^8)/(1 - (-0.5)) = 7(1 - 1/256)/(1.5) = 7(255/256)/(3/2) = 7(255/256)(2/3) = 7(510/768) = 3570/768 = 595/128 = 119/16. So the sum of the first 8 terms is 119/16. Choice B correctly applies the formula with a = 7, r = -0.5, n = 8 and calculates the sum as 119/16 with accurate arithmetic. Choice A gives 1799/128, which appears to use a different calculation—perhaps an error in computing (-0.5)^8 = 1/256 or in the subsequent arithmetic; with negative ratios, sign errors can compound quickly. Common pitfall: confusing n (number of terms) with the last exponent. In a + ar + ar² + ... + ar^(n-1), there are n terms, but the last exponent is (n-1)! Count carefully: if the series is 'first 5 terms,' then n = 5, but the last term is ar^4, not ar^5. Off-by-one errors in n throw off the sum. The formula already accounts for this with r^n, not r^(n-1).

Question 9

Use $S_n = \frac{a(1-r^n)}{1-r}$ to calculate the sum $3 + 3(1.1) + 3(1.1)^2 + \cdots + 3(1.1)^9.$ Here $a=3$ , $r=1.1$ , and $n=10$ .

$\displaystyle 3\,\frac{1-1.1^{10}}{1-1.1}$
$\displaystyle 3\,\frac{1-1.1^{10}}{1+1.1}$
$\displaystyle 3\,\frac{1-1.1^{9}}{1-1.1}$
$\displaystyle 3\,\frac{1.1^{10}-1}{1-1.1}$

Explanation: This question tests your understanding of geometric series—the sum of terms from a geometric sequence—and how to use the formula

S_n = a(1 - r^n)/(1 - r)

to calculate these sums efficiently. To use the formula: (1) identify the first term a (what's the first number being added?), (2) find the common ratio r (divide any term by the previous term), (3) count how many terms n you're summing, (4) substitute into

S_n = a(1 - r^n)/(1 - r)

, (5) calculate carefully. Example: for 5 + 10 + 20 + 40 + 80, we have a = 5, r = 2, n = 5, so

S_5 = 5(1 - 2^5)/(1 - 2) = 5(1 - 32)/(-1) = 5(-31)/(-1) = 155

. For the series 3 + 3(1.1) + 3(1.1)^2 + ... + 3(1.1)^9 with a = 3, r = 1.1, n = 10: Using the formula

S_n = a(1 - r^n)/(1 - r)

, we substitute:

S_10 = 3(1 - 1.1^{10})/(1 - 1.1)

, which is the expression needed. So the sum of the first 10 terms is given by that formula. Choice A correctly applies the formula with a = 3, r = 1.1, n = 10 with accurate parameters. Choice B identifies the parameters wrong by using n=9 instead of n=10; the exponents go from 0 to 9, which is 10 terms, so n=10, not n-1. Common pitfall: confusing n (number of terms) with the last exponent. In

a + ar + ar^2 + \dots + ar^{n-1}

, there are n terms, but the last exponent is (n-1)! Count carefully: if the series is 'first 10 terms,' then n = 10, but the last term is ar^9, not ar^{10}. The formula already accounts for this with r^n, not r^(n-1).

Question 10

What is the sum of the first 8 terms of the geometric series $7+14+28+\cdots$ ? (So $a=7$ , $r=2$ , $n=8$ .)

$896$
$1024$
$1792$
$1785$

Explanation: This question tests your understanding of geometric series—the sum of terms from a geometric sequence—and how to use the formula S_n = a(1 - r^n)/(1 - r) to calculate these sums efficiently. To use the formula: (1) identify the first term a (what's the first number being added?), (2) find the common ratio r (divide any term by the previous term), (3) count how many terms n you're summing, (4) substitute into S_n = a(1 - r^n)/(1 - r), (5) calculate carefully. For the series 7 + 14 + 28 + ... with a = 7, r = 2, n = 8: Using the formula S_8 = 7(1 - 2^8)/(1 - 2) = 7(1 - 256)/(-1) = 7(-255)/(-1) = 7(255) = 1785. So the sum of the first 8 terms is 1785. Choice B correctly applies the formula with a = 7, r = 2, n = 8 and calculates the sum as 1785 with accurate arithmetic. Choice A forgets the (1 - r^n) is negative when r > 1: if r = 2 and n = 8, then r^n = 256, so (1 - 256) = -255 (negative!). Combined with (1 - r) = 1 - 2 = -1 (also negative), we get (-255)/(-1) = 255, then *7=1785 (positive). Two negatives make a positive—don't lose those signs! Why this formula is powerful: to sum 2 + 6 + 18 + 54 + ... + (2·3^99), you'd need to add 100 terms manually (impossible!). With the formula: S_100 = 2(1 - 3^100)/(1 - 3) and you're done (calculator handles 3^100). The formula turns a hundred-operation problem into a few operations. That's the beauty of having a formula!

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Algebra

Algebra Help: Deriving Applying The Geometric Series Formula

Review real example questions for Deriving Applying The Geometric Series Formula in Algebra.

Question 1

$S_n = \frac{a(1-r^n)}{1-r}$
$S_n = a + (n-1)r$
$S_n = \frac{a(1-r^{n-1})}{1-r}$
$S_n = \frac{a(1-r^n)}{1+r}$

Explanation: This question tests your understanding of geometric series—the sum of terms from a geometric sequence—and how to use the formula

S_n = a(1 - r^n)/(1 - r)

to calculate these sums efficiently. The geometric series formula

S_n = a(1 - r^n)/(1 - r)

comes from a clever trick: write the sum

S_n = a + ar + ar^2 + \cdots + ar^{n-1}

, then multiply by

r

to get

rS_n = ar + ar^2 + ar^3 + \cdots + ar^n

. Subtracting these (

S_n - rS_n

) makes almost all terms cancel, leaving just

S_n(1 - r) = a(1 - r^n)

, so

S_n = a(1 - r^n)/(1 - r)

. The middle terms canceling is the magic that makes this work! Deriving

S_n = a(1 - r^n)/(1 - r)

: (1) Write the sum:

S_n = a + ar + ar^2 + \cdots + ar^{n-1}

. (2) Multiply both sides by

r

rS_n = ar + ar^2 + ar^3 + \cdots + ar^n

. (3) Subtract second from first:

S_n - rS_n = (a + ar + ar^2 + \cdots + ar^{n-1}) - (ar + ar^2 + ar^3 + \cdots + ar^n)

. (4) Notice the cancellation: all middle terms cancel, leaving

S_n - rS_n = a - ar^n

. (5) Factor left side:

S_n(1 - r) = a(1 - r^n)

. (6) Divide by

(1 - r)

S_n = a(1 - r^n)/(1 - r)

r^{n-1}

instead of

r^n

in the numerator; this happens if you miss that the remaining term after cancellation is

-ar^n

from the shifted series. Derivation memory aid: the trick is writing

S_n

, then writing

rS_n

(shifted one term), then subtracting. When you subtract, the middle terms align and cancel:

S_n

has 'ar' and

rS_n

has 'ar' (opposite signs, cancel!),

S_n

has 'ar^2

and

rS_n

has 'ar^2' (cancel!), etc. Only a from

S_n

and

-ar^n

from

rS_n$ don't cancel. This telescoping is the insight! Once you see it, you'll never forget the derivation.

Question 2

For the geometric series $2 + 6 + 18 + 54$ , identify $a$ , $r$ , and $n$ , then find the sum using $S_n = \frac{a(1-r^n)}{1-r}$ .

$a=2, r=2, n=4; S_4=30$
$a=2, r=3, n=4; S_4=80$
$a=2, r=3, n=3; S_3=26$
$a=6, r=3, n=4; S_4=240$

Explanation: This question tests your understanding of geometric series—the sum of terms from a geometric sequence—and how to use the formula

S_n = \frac{a(1 - r^n)}{1 - r}

S_n = \frac{a(1 - r^n)}{1 - r}

S_4 = 2(1 - 3^4)/(1 - 3) = 2(1 - 81)/(-2) = 2(-80)/(-2) = 80

S_100 = 2(1 - 3^{100})/(1 - 3)

and you're done (calculator handles 3^100). The formula turns a hundred-operation problem into a few operations. That's the beauty of having a formula!

Question 3

What is the sum of the first 5 terms of the geometric series $2+6+18+54+162?$ (Identify $a=2$ , $r=3$ , $n=5$ .)

$242$
$80$
$121$
$244$

Question 4

A bouncing ball rises to $80\%$ of its previous height after each bounce. If the first bounce reaches $10$ ft, what is the total of the bounce heights for the first $5$ bounces?

Sum the geometric series $10+10(0.8)+10(0.8)^2+10(0.8)^3+10(0.8)^4$ (so $a=10$ , $r=0.8$ , $n=5$ ).

$\dfrac{10(1-0.8^5)}{1-0.8}=33.616$
$\dfrac{10(1-0.8^4)}{1-0.8}=29.52$
$\dfrac{10(1-0.8^5)}{1-0.8}=16.808$
$10(0.8)^5=3.2768$

Explanation: This question tests your understanding of geometric series—the sum of terms from a geometric sequence—and how to use the formula S_n = a(1 - r^n)/(1 - r) to calculate these sums efficiently. Geometric series appear in finance: when calculating mortgage payments, the total amount paid is a geometric series with the interest rate as the ratio. Each payment accumulates interest, creating the geometric pattern. The formula lets you calculate loan payoffs, investment growth with regular contributions, and other real-world money situations where compounding occurs! In the context of a bouncing ball, each bounce height is 80% of the previous: 10 ft, then 10(0.8) = 8 ft, then 10(0.8)² = 6.4 ft, creating a geometric series. With a = 10, r = 0.8, n = 5, the total is S_5 = 10(1 - (0.8)^5)/(1 - 0.8) = 10(1 - 0.32768)/(0.2) = 10(0.67232)/(0.2) = 10(3.3616) = 33.616. This shows the total distance traveled upward in 5 bounces. Choice A correctly applies the formula with a = 10, r = 0.8, n = 5 and calculates the sum as 33.616 with accurate arithmetic. Choice C has a calculation error: it shows 16.808 which is exactly half of 33.616, suggesting they might have divided by 0.2 incorrectly or made an arithmetic mistake. When 0 < r < 1, both (1 - r^n) and (1 - r) are positive, so the sum should be positive and match our calculation. Why this formula is powerful: to sum 2 + 6 + 18 + 54 + ... + (2·3^99), you'd need to add 100 terms manually (impossible!). With the formula: S_100 = 2(1 - 3^100)/(1 - 3) and you're done (calculator handles 3^100). The formula turns a hundred-operation problem into a few operations. That's the beauty of having a formula!

Question 5

Use the geometric series formula $S_n=\dfrac{a(1-r^n)}{1-r}$ to calculate the sum $3+3(1.1)+3(1.1)^2+\cdots+3(1.1)^9.$ (So $a=3$ , $r=1.1$ , $n=10$ .)

$\dfrac{3\left(1-1.1^{9}\right)}{1-1.1}$
$\dfrac{3\left(1-10\cdot 1.1\right)}{1-1.1}$
$\dfrac{3\left(1-1.1^{10}\right)}{1-1.1}$
$\dfrac{3\left(1-1.1^{10}\right)}{1.1}$

Explanation: This question tests your understanding of geometric series—the sum of terms from a geometric sequence—and how to use the formula S_n = a(1 - r^n)/(1 - r) to calculate these sums efficiently. To use the formula: (1) identify the first term a (what's the first number being added?), (2) find the common ratio r (divide any term by the previous term), (3) count how many terms n you're summing, (4) substitute into S_n = a(1 - r^n)/(1 - r), (5) calculate carefully. Example: for 5 + 10 + 20 + 40 + 80, we have a = 5, r = 2, n = 5, so S_5 = 5(1 - 2^5)/(1 - 2) = 5(1 - 32)/(-1) = 5(-31)/(-1) = 155. For the series 3 + 3(1.1) + 3(1.1)² + ... + 3(1.1)⁹ with a = 3, r = 1.1, n = 10: Using the formula S_n = a(1 - r^n)/(1 - r), we substitute: S_10 = 3(1 - 1.1^10)/(1 - 1.1). This matches the expression in choice A exactly. So the sum is expressed as 3(1 - 1.1^10)/(1 - 1.1). Choice A correctly applies the formula with a = 3, r = 1.1, n = 10 with accurate substitution. Choice B has n = 9 instead of n = 10: when counting from 3(1.1)⁰ to 3(1.1)⁹, that's 10 terms total (including the 0th power). Remember that powers go from 0 to n-1, giving n terms total. Off-by-one errors in counting terms are common! Common pitfall: confusing n (number of terms) with the last exponent. In a + ar + ar² + ... + ar^(n-1), there are n terms, but the last exponent is (n-1)! Count carefully: if the series is 'first 5 terms,' then n = 5, but the last term is ar^4, not ar^5. Off-by-one errors in n throw off the sum. The formula already accounts for this with r^n, not r^(n-1).

Question 6

This is the geometric series $10+10(0.8)+10(0.8)^2+10(0.8)^3+10(0.8)^4$ with $a=10$ , $r=0.8$ , $n=5$ .

$\approx 33.62$
$\approx 40.00$
$\approx 26.89$
$\approx 36.16$

Explanation: This question tests your understanding of geometric series—the sum of terms from a geometric sequence—and how to use the formula S_n = a(1 - r^n)/(1 - r) to calculate these sums efficiently. Geometric series appear in finance: when calculating mortgage payments, the total amount paid is a geometric series with the interest rate as the ratio. Each payment accumulates interest, creating the geometric pattern. The formula lets you calculate loan payoffs, investment growth with regular contributions, and other real-world money situations where compounding occurs! In the context of ball rebound heights, each height is 80% of the previous, creating a geometric series: 10 + 10(0.8) + 10(0.8)^2 + 10(0.8)^3 + 10(0.8)^4. With a = 10, r = 0.8, n = 5, the total is S_5 = 10(1 - 0.8^5)/(1 - 0.8) ≈ 10(1 - 0.32768)/0.2 ≈ 10(0.67232)/0.2 ≈ 10(3.3616) ≈ 33.62. This shows the total height summed over the first 5 bounces. Choice A correctly applies the formula with a = 10, r = 0.8, n = 5 and calculates the sum as ≈33.62 with accurate arithmetic. Choice D uses the arithmetic series formula instead of the geometric series formula: arithmetic series use S_n = n(a + last term)/2 (for constant difference), but this is a geometric series with constant ratio, needing S_n = a(1 - r^n)/(1 - r). Don't confuse the two types! For financial applications like mortgages: the payment P, interest rate r (per period), and number of periods n plug into formulas built from geometric series. You might see S = P((1+r)^n - 1)/r or similar—these come from rearranging the geometric series formula! Understanding the underlying geometric series helps you understand why mortgage formulas look the way they do.

Question 7

$\$ 1{,}200.00$
$\$ 1{,}236.94$
$\$ 1{,}260.00$
$\$ 1{,}230.79$

100 grows by 0.5% per month, creating a geometric series: 100(1.005)^11 + 100(1.005)^10 + ... + 100(1.005)^0. With a = 100, r = 1.005, n = 12, the total is S_12 = 100(1 - 1.005^12)/(1 - 1.005) = 100(1 - 1.061677812)/(−0.005) = 100(−0.061677812)/(−0.005) = 100(12.3355624) = 1233.56. Wait, let me recalculate: the series goes from 1.005^11 down to 1.005^0, which we can rewrite as 100(1.005^0 + 1.005^1 + ... + 1.005^11). Using a = 100, r = 1.005, n = 12: S_12 = 100(1.005^12 - 1)/(1.005 - 1) = 100(1.061677812 - 1)/0.005 = 100(0.061677812)/0.005 = 100(12.3355624) = 1236.94. This shows the account value after 12 deposits is

1,236.94. Choice C correctly calculates the sum as

1,236.94 with accurate arithmetic and proper handling of the compound interest. Choice B gives

Question 8

Use $S_n=\dfrac{a(1-r^n)}{1-r}$ to find the sum of the geometric series $7+7(-0.5)+7(-0.5)^2+\cdots+7(-0.5)^7.$ (So $a=7$ , $r=-0.5$ , $n=8$ .)

$\dfrac{120}{16}$
$\dfrac{1799}{128}$
$\dfrac{119}{16}$
$\dfrac{63}{8}$

Explanation: This question tests your understanding of geometric series—the sum of terms from a geometric sequence—and how to use the formula S_n = a(1 - r^n)/(1 - r) to calculate these sums efficiently. A geometric series is the sum of terms from a geometric sequence: if the sequence is 2, 6, 18, 54, ... (multiply by 3 each time), the series is 2 + 6 + 18 + 54 + ... (adding those terms up). Rather than adding manually (tedious for many terms!), we use the formula S_n = a(1 - r^n)/(1 - r), where a is the first term, r is the common ratio, and n is how many terms we're summing. This formula works for any finite geometric series! For the series 7 + 7(-0.5) + 7(-0.5)^2 + ... + 7(-0.5)^7 with a = 7, r = -0.5, n = 8: Using the formula S_n = a(1 - r^n)/(1 - r), we substitute: S_8 = 7(1 - (-0.5)^8)/(1 - (-0.5)) = 7(1 - 1/256)/(1.5) = 7(255/256)/(3/2) = 7(255/256)(2/3) = 7(510/768) = 3570/768 = 595/128 = 119/16. So the sum of the first 8 terms is 119/16. Choice B correctly applies the formula with a = 7, r = -0.5, n = 8 and calculates the sum as 119/16 with accurate arithmetic. Choice A gives 1799/128, which appears to use a different calculation—perhaps an error in computing (-0.5)^8 = 1/256 or in the subsequent arithmetic; with negative ratios, sign errors can compound quickly. Common pitfall: confusing n (number of terms) with the last exponent. In a + ar + ar² + ... + ar^(n-1), there are n terms, but the last exponent is (n-1)! Count carefully: if the series is 'first 5 terms,' then n = 5, but the last term is ar^4, not ar^5. Off-by-one errors in n throw off the sum. The formula already accounts for this with r^n, not r^(n-1).

Question 9

Use $S_n = \frac{a(1-r^n)}{1-r}$ to calculate the sum $3 + 3(1.1) + 3(1.1)^2 + \cdots + 3(1.1)^9.$ Here $a=3$ , $r=1.1$ , and $n=10$ .

$\displaystyle 3\,\frac{1-1.1^{10}}{1-1.1}$
$\displaystyle 3\,\frac{1-1.1^{10}}{1+1.1}$
$\displaystyle 3\,\frac{1-1.1^{9}}{1-1.1}$
$\displaystyle 3\,\frac{1.1^{10}-1}{1-1.1}$

Explanation: This question tests your understanding of geometric series—the sum of terms from a geometric sequence—and how to use the formula

S_n = a(1 - r^n)/(1 - r)

S_n = a(1 - r^n)/(1 - r)

, (5) calculate carefully. Example: for 5 + 10 + 20 + 40 + 80, we have a = 5, r = 2, n = 5, so

S_5 = 5(1 - 2^5)/(1 - 2) = 5(1 - 32)/(-1) = 5(-31)/(-1) = 155

. For the series 3 + 3(1.1) + 3(1.1)^2 + ... + 3(1.1)^9 with a = 3, r = 1.1, n = 10: Using the formula

S_n = a(1 - r^n)/(1 - r)

, we substitute:

S_10 = 3(1 - 1.1^{10})/(1 - 1.1)

a + ar + ar^2 + \dots + ar^{n-1}

Question 10

What is the sum of the first 8 terms of the geometric series $7+14+28+\cdots$ ? (So $a=7$ , $r=2$ , $n=8$ .)

$896$
$1024$
$1792$
$1785$

Explanation: This question tests your understanding of geometric series—the sum of terms from a geometric sequence—and how to use the formula S_n = a(1 - r^n)/(1 - r) to calculate these sums efficiently. To use the formula: (1) identify the first term a (what's the first number being added?), (2) find the common ratio r (divide any term by the previous term), (3) count how many terms n you're summing, (4) substitute into S_n = a(1 - r^n)/(1 - r), (5) calculate carefully. For the series 7 + 14 + 28 + ... with a = 7, r = 2, n = 8: Using the formula S_8 = 7(1 - 2^8)/(1 - 2) = 7(1 - 256)/(-1) = 7(-255)/(-1) = 7(255) = 1785. So the sum of the first 8 terms is 1785. Choice B correctly applies the formula with a = 7, r = 2, n = 8 and calculates the sum as 1785 with accurate arithmetic. Choice A forgets the (1 - r^n) is negative when r > 1: if r = 2 and n = 8, then r^n = 256, so (1 - 256) = -255 (negative!). Combined with (1 - r) = 1 - 2 = -1 (also negative), we get (-255)/(-1) = 255, then *7=1785 (positive). Two negatives make a positive—don't lose those signs! Why this formula is powerful: to sum 2 + 6 + 18 + 54 + ... + (2·3^99), you'd need to add 100 terms manually (impossible!). With the formula: S_100 = 2(1 - 3^100)/(1 - 3) and you're done (calculator handles 3^100). The formula turns a hundred-operation problem into a few operations. That's the beauty of having a formula!