PSAT Math : How to find patterns in exponents

Study concepts, example questions & explanations for PSAT Math

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Example Questions

Example Question #251 : Exponents

If ax·a4 = a12 and (by)3 = b15, what is the value of x - y?

Possible Answers:

-4

-9

-2

6

3

Correct answer:

3

Explanation:

Multiplying like bases means add the exponents, so x+4 = 12, or x = 8.

Raising a power to a power means multiply the exponents, so 3y = 15, or y = 5.

x - y = 8 - 5 = 3.

Example Question #252 : Exponents

If p and q are positive integrers and 27= 9q, then what is the value of q in terms of p?

Possible Answers:

3p

p

(3/2)p

2p

(2/3)p

Correct answer:

(3/2)p

Explanation:

The first step is to express both sides of the equation with equal bases, in this case 3. The equation becomes 33p = 32q. So then 3p = 2q, and q = (3/2)p is our answer. 

Example Question #3 : How To Find Patterns In Exponents

Simplify 272/3.

Possible Answers:

3

125

9

729

27

Correct answer:

9

Explanation:

272/3 is 27 squared and cube-rooted. We want to pick the easier operation first. Here that is the cube root. To see that, try both operations. 

272/3 = (272)1/3 = 7291/3 OR

272/3 = (271/3)2 = 32

Obviously 32 is much easier. Either 32 or 7291/3 will give us the correct answer of 9, but with 32 it is readily apparent. 

Example Question #251 : Exponents

If \(\displaystyle a\) and \(\displaystyle b\) are integers and 

\(\displaystyle \left ( \frac{1}{3} \right )^{a}=27^{b}\) 

what is the value of \(\displaystyle a\div b\)? 

Possible Answers:

\(\displaystyle 3\)

\(\displaystyle -3\)

\(\displaystyle 9\)

\(\displaystyle -\frac{1}{3}\)

\(\displaystyle \frac{1}{3}\)

Correct answer:

\(\displaystyle -3\)

Explanation:

To solve this problem, we will have to take the log of both sides to bring down our exponents. By doing this, we will get \dpi{100} \small a\ast log\left (\frac{1}{3} \right )= b\ast log\left ( 27 \right )\(\displaystyle \dpi{100} \small a\ast log\left (\frac{1}{3} \right )= b\ast log\left ( 27 \right )\).

To solve for \dpi{100} \small \frac{a}{b}\(\displaystyle \dpi{100} \small \frac{a}{b}\) we will have to divide both sides of our equation by \dpi{100} \small log\frac{1}{3}\(\displaystyle \dpi{100} \small log\frac{1}{3}\) to get \dpi{100} \small \frac{a}{b}=\frac{log\left ( 27 \right )}{log\left ( \frac{1}{3} \right )}\(\displaystyle \dpi{100} \small \frac{a}{b}=\frac{log\left ( 27 \right )}{log\left ( \frac{1}{3} \right )}\).

\dpi{100} \small \frac{log\left ( 27 \right )}{log\left ( \frac{1}{3} \right )}\(\displaystyle \dpi{100} \small \frac{log\left ( 27 \right )}{log\left ( \frac{1}{3} \right )}\) will give you the answer of –3.

Example Question #5 : How To Find Patterns In Exponents

If\(\displaystyle \log 2=0.301\) and \(\displaystyle \log 3=0.477\), then what is \(\displaystyle \log 12\)?

Possible Answers:

\(\displaystyle 1.116\)

\(\displaystyle 1.592\)

\(\displaystyle 1.346\)

\(\displaystyle 1.079\)

\(\displaystyle 1.255\)

Correct answer:

\(\displaystyle 1.079\)

Explanation:

We use two properties of logarithms: 

log(xy) = log (x) + log (y)\(\displaystyle log(xy) = log (x) + log (y)\)

log(x^{n}) = nlog (x)\(\displaystyle log(x^{n}) = nlog (x)\)

So \(\displaystyle \log 12=2 \log2+\log3\)

Example Question #6 : How To Find Patterns In Exponents

Evaluate:

x^{-3}x^{6}\(\displaystyle x^{-3}x^{6}\)

Possible Answers:

x^{6}\(\displaystyle x^{6}\)

x^{9}\(\displaystyle x^{9}\)

x^{3}\(\displaystyle x^{3}\)

x^{-18}\(\displaystyle x^{-18}\)

x^{-3}\(\displaystyle x^{-3}\)

Correct answer:

x^{3}\(\displaystyle x^{3}\)

Explanation:

x^{m}\ast x^{n} = x^{m + n}\(\displaystyle x^{m}\ast x^{n} = x^{m + n}\), here \(\displaystyle m=-3\) and \(\displaystyle n=6\), hence \(\displaystyle -3+6=3\).

Example Question #7 : How To Find Patterns In Exponents

Solve for \(\displaystyle x\)

\left ( \frac{2}{3} \right )^{x+1} = \frac{27}{8}\(\displaystyle \left ( \frac{2}{3} \right )^{x+1} = \frac{27}{8}\)

Possible Answers:

None of the above

\(\displaystyle 2\)

\(\displaystyle -4\)

\(\displaystyle 4\)

\(\displaystyle -2\)

Correct answer:

\(\displaystyle -4\)

Explanation:

\left ( \frac{2}{3} \right )^{x+1} = \frac{27}{8}\(\displaystyle \left ( \frac{2}{3} \right )^{x+1} = \frac{27}{8}\) = \left ( \frac{3}{2} \right )^{3} = \left ( \frac{2}{3} \right )^{-3}\(\displaystyle \left ( \frac{3}{2} \right )^{3} = \left ( \frac{2}{3} \right )^{-3}\)

 \(\displaystyle x+1=-3\) which means \(\displaystyle x=-4\)

Example Question #8 : How To Find Patterns In Exponents

Which of the following statements is the same as:

\(\displaystyle \dfrac{4^{x+2}}{2^y}\)

\(\displaystyle I)\ 2^{2x+4-y} \quad II)\ 2^{x+2} / 4^y \quad III)\ (4^x)( 4^2 )(2^{-y} )\)

Possible Answers:

\(\displaystyle II \mbox{ and } III \mbox{ only}\)

\(\displaystyle I \mbox{ and } III \mbox{ only}\)

\(\displaystyle I \mbox{ only}\)

\(\displaystyle I, II \mbox{ and } III\)

\(\displaystyle I \mbox{ and } II \mbox{ only}\)

Correct answer:

\(\displaystyle I \mbox{ and } III \mbox{ only}\)

Explanation:

Remember the laws of exponents. In particular, when the base is nonzero:

\(\displaystyle b^{m+n} = b^m b^n \qquad (b^m)^n = b^{mn} \\ (b \times c)^n = b^n c^n \qquad b^{-n} = 1/b^n\)

An effective way to compare these statements, is to convert them all into exponents with base 2. The original statement becomes:

\(\displaystyle 4^{x+2} / 2^y = 4^{x+2} (2^{-y}) = (2^2)^{x+2} 2^{-y} = 2^{2(x+2) } 2^{-y} = 2^{2x+4-y}\)

This is identical to statement I. Now consider statement II:

\(\displaystyle 2^{x+2} / 4^y = 2^{x+2} ( 4^{-y} ) = 2^{x+2} ( 2^2 )^{-y} = 2^{x+2} 2^{-2y} = 2^{x+2-2y} \neq 2^{2x+4-y}\)

Therefore, statement II is not identical to the original statement. Finally, consider statement III:

\(\displaystyle (4^x)(4^2 ) (2^{-y}) = (2^2)^x (2^2)^2 (2^{-y}) = (2^{2x} )( 2^4 ) (2^{-y}) = 2^{2x+4-y}\)

which is also identical to the original statement. As a result, only I and III are the same as the original statement. 

Example Question #1 : How To Find Patterns In Exponents

Write in exponential form:

\(\displaystyle \sqrt[3]{\left ( x-1 \right )^{2}}\)

Possible Answers:

\(\displaystyle \left ( x-1 \right )^{3}\)

\(\displaystyle \left ( x-1 \right )\)

\(\displaystyle \left ( x-1 \right )^{2}\)

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

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

Correct answer:

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

Explanation:

Using properties of radicals e.g., \(\displaystyle \sqrt[m]{x^{n}} = x^{\frac{n}{m}}\)

we get \(\displaystyle \sqrt[3]{\left ( x-1 \right )^{2}} = \left ( x-1 \right )^{\frac{2}{3}}\)

Example Question #10 : How To Find Patterns In Exponents

Write in exponential form:

\(\displaystyle \sqrt[3]{x^{5}}\)

Possible Answers:

\(\displaystyle x^{3}\)

\(\displaystyle x^{\frac{5}{3}}\)

\(\displaystyle x^{\frac{3}{5}}\)

\(\displaystyle x^{5}\)

\(\displaystyle x^{5-3}\)

Correct answer:

\(\displaystyle x^{\frac{5}{3}}\)

Explanation:

Properties of Radicals

\(\displaystyle \sqrt[]{x} = x^{\frac{1}{2}}\)

\(\displaystyle \sqrt[3]{x} = x^{\frac{1}{3}}\)

\(\displaystyle x^{\frac{5}{3}}\)

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