PSAT Math : Sequences

Study concepts, example questions & explanations for PSAT Math

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Example Questions

Example Question #6 : How To Find The Answer To An Arithmetic Sequence

Brad can walk 3600 feet in 10 minutes. How many yards can he walk in ten seconds?

Possible Answers:

10

30

15

20

25

Correct answer:

20

Explanation:

If Brad can walk 3600 feet in 10 minutes, then he can walk 3600/10 = 360 feet per minute, and 360/60 = 6 feet per second.

There are 3 feet in a yard, so Brad can walk 6/3 = 2 yards per second, or 2 x 10 = 20 yards in 10 seconds.

 

Example Question #1 : How To Find The Answer To An Arithmetic Sequence

If the sequence above continues as shown, what is the sum of the first 18 terms?

Possible Answers:

Correct answer:

Explanation:

The simplest way to handle this question is to consider the sequence as being a set of 3 repeating terms, (3,2,4). Within the first 18 terms in the sequence, this pattern will repeat a total of 6 times.

The sum of one repeat is:

Since there are 6 repeats, take the sum of one repeat (9) and multiply by the number of repeats:

The sum of the first 18 terms is 54.

Example Question #2 : How To Find The Answer To An Arithmetic Sequence

Possible Answers:

Correct answer:

Explanation:

Shortcut:  If you add up al the integers from  to , the sum is zero because

the positives and negatives all cancel out.  Start with the next number:

Example Question #1 : How To Find The Nth Term Of An Arithmetic Sequence

2, 8, 14, 20

The first term in the sequence is 2, and each following term is determined by adding 6. What is the value of the 50th term?

 

 

Possible Answers:

302

320

300

296

Correct answer:

296

Explanation:

We start by multiplying 6 times 46, since the first 4 terms are already listed. We then add the product, 276, to the last listed term, 20. This gives us our answer of 296.

 

 

Example Question #2 : How To Find The Nth Term Of An Arithmetic Sequence

Which of the following could not be a term in the sequence 5, 10, 15, 20...?

Possible Answers:

2500

10005

3751

35

Correct answer:

3751

Explanation:

All answers in the sequence must end in a 5 or a 0.

Example Question #3 : Nth Term Of An Arithmetic Sequence

In an arithmetic sequence, each term is two greater than the one that precedes it. If the sum of the first five terms of the sequence is equal to the difference between the first and fifth terms, what is the tenth term of the sequence?

Possible Answers:

10

15.6

0.6

2.4

–0.6

 

Correct answer:

15.6

Explanation:

Let a1 represent the first term of the sequence and an represent the nth term.

We are told that each term is two greater than the term that precedes it. Thus, we can say that:

a2 = a1 + 2

a3 = a1 + 2 + 2 = a1 + 2(2)

a4 = a1 + 3(2)

a5 = a1 + 4(2)

an = a1 + (n-1)(2)

The problem tells us that the sum of the first five terms is equal to the difference between the fifth and first terms. Let's write an expression for the sum of the first five terms.

sum = a1 + (a1 + 2) + (a1 + 2(2)) + (a1 + 3(2)) + (a1 + 4(2))

= 5a1 + 2 + 4 + 6 + 8

= 5a1 + 20

Next, we want to write an expression for the difference between the fifth and first terms.

a5 - a1 = a1 + 4(2) – a1 = 8

Now, we set the two expressions equal and solve for a1.

5a1 + 20 = 8

Subtract 20 from both sides.

5a1 = –12

a1 = –2.4.

The question ultimately asks us for the tenth term of the sequence. Now, that we  have the first term, we can find the tenth term.

a10 = a1 + (10 – 1)(2)

a10 = –2.4 + 9(2)

= 15.6

The answer is 15.6 .

Example Question #1 : How To Find The Nth Term Of An Arithmetic Sequence

In a certain sequence, an+1 = (an)2 – 1, where an represents the nth term in the sequence. If the third term is equal to the square of the first term, and all of the terms are positive, then what is the value of (a2)(a3)(a4)?

Possible Answers:

6

24

48

63

72

Correct answer:

48

Explanation:

Let a1 be the first term in the sequence. We can use the fact that an+1 = (an)2 – 1 in order to find expressions for the second and third terms of the sequence in terms of a1.

a2 = (a1)2 – 1

a3 = (a2)2 – 1 = ((a1)2 – 1)2 – 1

We can use the fact that, in general, (a – b)2 = a2 – 2abb2 in order to simplify the expression for a3.

a= ((a1)2 – 1)2 – 1

= (a1)4 – 2(a1)2 + 1 – 1 = (a1)4 – 2(a1)2

We are told that the third term is equal to the square of the first term. 

a3 = (a1)2

We can substitute (a1)4 – 2(a1)for a3.

(a1)4 – 2(a1)= (a1)2

Subtract (a1)2 from both sides.

(a1)4 – 3(a1)= 0

Factor out (a1)from both terms.

(a1)2 ((a1)2 – 3) = 0

This means that either (a1)= 0, or (a1)2 – 3 = 0. 

If (a1)= 0, then a1 must be 0. However, we are told that all the terms of the sequence are positive. Therefore, the first term can't be 0.

Next, let's solve (a1)2 – 3 = 0.

Add 3 to both sides.

(a1)= 3

Take the square root of both sides. 

a1 = ±√3

However, since all the terms are positive, the only possible value for a1 is √3.

Now, that we know that a1 = √3, we can find a2, a3, and a4.

a2 = (a1)2 – 1 = (√3)2 – 1 = 3 – 1 = 2

a3 = (a2)2 – 1 = 2– 1 = 4 – 1 = 3

a4 = (a3)2 – 1 = 32 – 1 = 9 – 1 = 8

The question ultimately asks for the product of the a2, a3, and a4, which would be equal to 2(3)(8), or 48.

The answer is 48.

Example Question #11 : Arithmetic Sequences

In the given sequence, the first term is 3 and each term after is one less than three times the previous term.

What is the sixth term in the sequence?

Possible Answers:

Correct answer:

Explanation:

The fourth term is: 3(23) – 1 = 69 – 1 = 68.

The fifth term is: 3(68) – 1 = 204 – 1 = 203.

The sixth term is: 3(203) – 1 = 609 – 1 = 608.

Example Question #1 : Nth Term Of An Arithmetic Sequence

Consider the following sequence of numbers:

What will be the 8th term in the sequence?

Possible Answers:

56

49

58

60

51

Correct answer:

51

Explanation:

Each number in the sequence in 7 more than the number preceding it.

The equation for the terms in an arithmetic sequence is an = a1 + d(n-1), where d is the difference.

The formula for the terms in this sequence is therefore an = 2 + 7(n-1). 

Plug in 8 for n to find the 8th term: 

a8 = 2 + 7(8-1) = 51

 

Example Question #381 : Arithmetic

The second and fourth terms of an arithmetic sequence are 9 and 18, respectively. What is its first term?

Possible Answers:

Correct answer:

Explanation:

The difference between the second and fourth terms of an arithmetic sequence is twice the common difference - or, equivalently, the common difference is half the difference between the second and fourth terms.

The common difference can be subtracted from the second term to obtain the first term:

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