PSAT Math : Exponential Operations

Study concepts, example questions & explanations for PSAT Math

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Example Questions

Example Question #4 : Exponents

Find the value of x such that:

8x-3 = 164-x

Possible Answers:

4

11/3

19/4

7/2

25/7

Correct answer:

25/7

Explanation:

In order to solve this equation, we first need to find a common base for the exponents. We know that 23 = 8 and 24 = 16, so it makes sense to use 2 as a common base, and then rewrite each side of the equation as a power of 2.

8x-3 = (23)x-3

We need to remember our property of exponents which says that (ab)c = abc.

Thus (23)x-3 = 23(x-3) = 23x - 9.

We can do the same thing with 164-x.

164-x = (24)4-x = 24(4-x) = 216-4x.

So now our equation becomes

23x - 9 = 216-4x

In order to solve this equation, the exponents have to be equal. 

3x - 9 = 16 - 4x

Add 4x to both sides.

7x - 9 = 16

Add 9 to both sides.

7x = 25

Divide by 7.

x = 25/7.

Example Question #5 : Exponents

Which of the following is equal to 410 + 410 + 410 + 410 + 411?

Possible Answers:

240

223

250

215

260

Correct answer:

223

Explanation:

We can start by rewriting 411 as 4 * 410. This will allow us to collect the like terms 410 into a single term.

410 + 410 + 410 + 410 + 411

= 410 + 410 + 410 + 410 + 4 * 410

= 8 * 410

Because the answer choices are written with a base of 2, we need to rewrite 8 and 4 using bases of two. Remember that 8 = 23, and 4 = 22.

8 * 410

= (23)(22)10

We also need to use the property of exponents that (ab)c = abc. We can rewrite (22)10 as 22x10 = 220.

(23)(22)10

= (23)(220)

Finally, we must use the property of exponents that a* ac = ab+c.

(23)(220) = 223

The answer is 223.

Example Question #1 : Exponential Operations

If 3 + 3n+3 = 81, what is 3n+2 ?

Possible Answers:

3

81

18

26

9

Correct answer:

26

Explanation:

3 + 3n+3 = 81

In this equation, there is a common factor of 3, which can be factored out.

Thus, 3(1 + 3n+2) = 81

Note: when 3 is factored out of 3n+3, the result is 3n+2 because (3n+3 = 31 * 3n+2). Remember that exponents are added when common bases are multiplied.  Also remember that 3 = 31.

3(1 + 3n+2) = 81

(1 + 3n+2) = 27

3n+2 = 26

Note: do not solve for n individually.  But rather seek to solve what the problem asks for, namely 3n+2.  

Example Question #7 : Exponents

If f(x) = (2 – x)(x/3), and 4n = f(10), then what is the value of n?

Possible Answers:

0

–2

5

2

–5

Correct answer:

5

Explanation:

First, let us use the definiton of f(x) to find f(10).

f(x) = (2 – x)(x/3)

f(10) = (2 – 10)(10/3)

= (–8)(10/3)

In order to evaluate the above expression, we can make use of the property of exponents that states that abc = (ab)c = (ac)b.

(–8)(10/3) = (–8)10(1/3) = ((–8)(1/3))10.

(–8)(1/3) requires us to take the cube root of –8. The cube root of –8 is –2, because (–2)3 = –8.

Let's go back to simplifying ((–8)(1/3))10.

((–8)(1/3))10 = (–2)10 = f(10)

We are asked to find n such that 4n = (–2)10. Let's rewrite 4n with a base of –2, because (–2)2 = 4.

4n = ((–2)2)n = (–2)2n = (–2)10

In order to (–2)2n = (–2)10, 2n must equal 10.

2n = 10

Divide both sides by 2.

n = 5.

The answer is 5.

Example Question #8 : Exponents

What is the value of n that satisfies the following equation?

2n·4n·8n·16 = 2-n·4-n·8-n

Possible Answers:

0

–1/3

–2/3

2/3

1/3

Correct answer:

–1/3

Explanation:

In order to solve this equation, we are going to need to use a common base. Because 2, 4, 8, and 16 are all powers of 2, we can rewrite both sides of the equation using 2 as a base. Since 22 = 4, 23 = 8, and 24 = 16, we can rewrite the original equation as follows:

2n4n8n16 = 2n 4n 8n

2n(22)n(23)n(24) = 2n(22)n(23)n

Now, we will make use of the property of exponents which states that (ab)c = abc.

2n(22n)(23n)(24) = 2n(22n)(23n)

Everything is now written as a power of 2. We can next apply the property of exponents which states that abac = ab+c.

2(n+2n+3n+4) = 2(–n + –2n + –3n)

We can now set the exponents equal and solve for n.

+ 2+ 3+ 4 = –n + –2n + –3n

Let's combine the n's on both sides.

6n + 4 = –6n

Add 6n to both sides.

12n + 4 = 0

Subtract 4 from both sides.

12n = –4

Divide both sides by 12.

n = –4/12 = –1/3

The answer is –1/3.

Example Question #9 : Exponents

If 1252x–4 = 6257–x, then what is the largest prime factor of x?

Possible Answers:

3

5

2

11

7

Correct answer:

2

Explanation:

First, we need to solve 1252x–4 = 6257–x . When solving equations with exponents, we usually want to get a common base. Notice that 125 and 625 both end in five. This means they are divisible by 5, and they could be both be powers of 5. Let's check by writing the first few powers of 5.

51 = 5

52 = 25

53 = 125

54 = 625

We can now see that 125 and 625 are both powers of 5, so let's replace 125 with 53 and 625 with 54

(53)2x–4 = (54)7–x

Next, we need to apply the rule of exponents which states that (ab)c = abc .

53(2x–4) = 54(7–x)

We now have a common base expressed with one exponent on each side. We must set the exponents equal to one another to solve for x.

3(2x – 4) = 4(7 – x)

Distribute the 3 on the left and the 4 on the right.

6x – 12 = 28 – 4x

Add 4x to both sides.

10x – 12 = 28

Add 12 to both sides.

10x = 40

Divide by 10 on both sides.

x = 4

However, the question asks us for the largest prime factor of x. The only factors of 4 are 1, 2, and 4. And of these, the only prime factor is 2. 

The answer is 2. 

Example Question #10 : Exponents

(x3)2 * x2

Possible Answers:

x4

x

x6

x4

x2

Correct answer:

x4

Explanation:

When an exponent is raised to a power, we multiply. But when two exponents with the same base are multiplied, we add them. So (x3)2 = x3*2 = x6. Then (x3)2 * x2 = x6 * x2 = x6 – 2 = x4.

Example Question #1141 : Psat Mathematics

If  \sqrt{x}=y^{2}=6, then which of the following is equivalent to x^3y^4?

Possible Answers:

6^7

6^6

6^4

6^{8}

6^8

Correct answer:

6^{8}

Explanation:

We can break up the equation into two smaller equations involving only x and y. Then, once we solve for x and y, we can find the value of x^3y^4.

Simp_sqrt1

Example Question #12 : Exponential Operations

If y^7x^8z^2 <0 which of the following must be true?

I. 

II. 

III. 

Possible Answers:

II

All

III

None

I

Correct answer:

None

Explanation:

 must be negative because it has an odd power and  and  have even powers above. But  and  could be positive or negative, so none of the scenarios has to be true.

Example Question #13 : Exponential Operations

-\left ( \frac{-1}{3} \right )^{2}

Possible Answers:

\frac{1}{9}

-\frac{1}{9}

\frac{2}{6}

\frac{1}{3}

-\frac{1}{3}

Correct answer:

-\frac{1}{9}

Explanation:

= \frac{\left ( -1 \right )^{2}}{\left ( 3 \right )^{2}}

= \frac{1}{9}

Hence the correct answer will be - \frac{1}{9}

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