ACT Math : Exponents

Study concepts, example questions & explanations for ACT Math

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Example Questions

Example Question #11 : Logarithms

What is the value of \(\displaystyle \textup{log}_2(64)\)?  Round to the nearest hundredth.

Possible Answers:

\(\displaystyle 7\)

\(\displaystyle 4.16\)

\(\displaystyle 1.81\)

\(\displaystyle 6\)

\(\displaystyle 4.5\)

Correct answer:

\(\displaystyle 6\)

Explanation:

You could solve this by using your calculator. Remember that you will have to translate this into:

\(\displaystyle \frac{log(64)}{log(2)}\)

Another way you can solve it is by noticing that \(\displaystyle 64=2^6\)

This means you can rewrite your logarithm:

\(\displaystyle log_2(2^6)\)

Applying logarithm rules, you can factor out the power:

\(\displaystyle log_2(2^6)=6log_2(2)\)

For any value \(\displaystyle n\)\(\displaystyle log_n(n) = 1\). Therefore, \(\displaystyle log_2(2) = 1\). So, your answer is \(\displaystyle 6\).

Example Question #101 : Exponents

Solve for \(\displaystyle x\)

\(\displaystyle 5^x^+^1=60\).

Round to the nearest hundredth.

Possible Answers:

\(\displaystyle 2.54\)

\(\displaystyle 1.54\)

\(\displaystyle 1.75\)

\(\displaystyle 4.73\)

\(\displaystyle 3.73\)

Correct answer:

\(\displaystyle 1.54\)

Explanation:

To solve an exponential equation like this, you need to use logarithms.  This can be translated into:

\(\displaystyle log_5(60)=x+1\)

Now, remember that your calculator needs to have this translated.  The logarithm \(\displaystyle log_5(60)\) is equal to the following:

\(\displaystyle \frac{log(60)}{log(5)}\), which equals approximately \(\displaystyle 2.54\).

Remember that you have the equation:

\(\displaystyle 2.54 = x + 1\)

Thus, \(\displaystyle x=1.54\).

Example Question #13 : Logarithms

Solve the following equation

\(\displaystyle 3^2^x^-^4=44\).

Possible Answers:

\(\displaystyle 7.31\)

\(\displaystyle 3.44\)

\(\displaystyle 3.72\)

\(\displaystyle 4\)

\(\displaystyle 2.11\)

Correct answer:

\(\displaystyle 3.72\)

Explanation:

In order to solve a question like this, you will need to use logarithms. First, start by converting this into a basic logarithm:

\(\displaystyle log_3(44)=2x-4\)

Recall that you need to convert \(\displaystyle log_3(44)\) for your calculator:

\(\displaystyle \frac{log(44)}{log(3)}\), which equals approximately \(\displaystyle 3.44\)

Thus, you can solve for \(\displaystyle x\):

\(\displaystyle 3.44=2x-4\)

\(\displaystyle 2x = 7.44\)

\(\displaystyle x=3.72\)

Example Question #2293 : Act Math

At the end of each year, an account compounds interest at a rate of \(\displaystyle 4.5\%\).  If the account began with \(\displaystyle \$1400\), how many years will it take for it to reach a value of \(\displaystyle \$5000\), presuming no withdrawals or deposits occur?

Possible Answers:

\(\displaystyle 29\)

\(\displaystyle 21\)

\(\displaystyle 39\)

\(\displaystyle 28\)

\(\displaystyle 40\)

Correct answer:

\(\displaystyle 29\)

Explanation:

The general function that defines this compounding interest is:

\(\displaystyle value = 1.045^t* 1400\), where \(\displaystyle t\) is the number of years.

What we are looking for is:

\(\displaystyle 1.045^t* 1400 = 5000\)

You can solve this using a logarithm.  First, isolate the variable term by dividing both sides:

\(\displaystyle 1.045^t =\frac{5000}{1400}\)

Which is:

\(\displaystyle 1.045^t =\frac{25}{7}\)

Next, recall that this is the logarithm:

\(\displaystyle t = log_1_._0_4_5(\frac{25}{7})\)

For this, you will need to do a base conversion:

\(\displaystyle log_1_._0_4_5(\frac{25}{7}) = \frac{log(\frac{25}{7})}{log(1.045)}\)

This is \(\displaystyle 28.9199397858296...\)

This means that it will take \(\displaystyle 29\) years.  \(\displaystyle 28\) is too few and at the end of \(\displaystyle 29\), you will have over \(\displaystyle 5000\).

Example Question #2294 : Act Math

What is the value of \(\displaystyle \textup{log}_4_._5(381)\)?  Round to the nearest hundredth.

Possible Answers:

\(\displaystyle 3.95\)

\(\displaystyle 2.58\)

\(\displaystyle 5.94\)

\(\displaystyle 8.14\)

\(\displaystyle 4.31\)

Correct answer:

\(\displaystyle 3.95\)

Explanation:

Remember that you will need to calculate your logarithm by doing a base conversion. This is done by changing \(\displaystyle log_4_._5(381)\) into:

\(\displaystyle \frac{log(381)}{log(4.5)}\)

Using your calculator, you can find this to be:

\(\displaystyle 3.95112604435386...\) or approximately \(\displaystyle 3.95\)

Example Question #101 : Exponents

if \(\displaystyle log_4(16) = x\), what is \(\displaystyle log_2(x)\)?

Possible Answers:

\(\displaystyle x = 4\)

\(\displaystyle x = 1\)

\(\displaystyle x = 2\)

\(\displaystyle x = 1/2\)

\(\displaystyle x = 0\)

Correct answer:

\(\displaystyle x = 1\)

Explanation:

The first step of this problem is to find

\(\displaystyle \log_4(16)\) by expanding to the formula

\(\displaystyle 4^y = 16.\)  

y is found to be 2.  The next step is to plug y in to the second log.  

\(\displaystyle log_2(2)\), which expands to

\(\displaystyle 2^x = 2,\)

\(\displaystyle x = 1\)

Example Question #102 : Exponents

Find  \(\displaystyle log_2(log_3(81))\).

Possible Answers:

\(\displaystyle x = 1/3\)

\(\displaystyle x = 1/2\)

\(\displaystyle x = 0\)

\(\displaystyle x = 2\)

\(\displaystyle x = 1\)

Correct answer:

\(\displaystyle x = 2\)

Explanation:

\(\displaystyle log_3(81)\) 

expands to

\(\displaystyle 3^y = 81, y = 4\)

\(\displaystyle log_2(4)\) 

expands to

\(\displaystyle 2^x = 4, x = 2\)

Example Question #11 : Logarithms

Simplify:

 \(\displaystyle \textup{log a}^2+\textup{log b}+\textup{log c}^3.\)

Possible Answers:

\(\displaystyle \textup{log}\left (a^2bc^3 \right )\)

\(\displaystyle \textup{log}\left (\frac{a^2b}{c} \right )\)

\(\displaystyle \textup{log}\left (\frac{2ab}{3c} \right )\)

\(\displaystyle \textup{log}\left (\frac{ab}{c} \right )\)

\(\displaystyle \textup{log}\left (6abc \right )\)

Correct answer:

\(\displaystyle \textup{log}\left (\frac{2ab}{3c} \right )\)

Explanation:

Here, we need to make use of some logarithm identities:  \(\displaystyle \textup{log}(\textup{a}^\textup{n})=\textup{n}*\textup{log(a)}log(a)+\textup{log}(\textup{b})=\textup{log(ab)}\)  

\(\displaystyle \textup{and } \textup{log(a)}-\textup{log(b)}=log(\frac{a}{b}).\)

Therefore, putting all of those things together, we get the final answer of \(\displaystyle \textup{log a}^2+\textup{log b}+\textup{log c}^3= \textup{log}(\frac{2\textup{ab}}{3\textup{c}}).\)

Example Question #11 : Logarithms

If

\(\displaystyle \log_{4}x = \frac{5}{2}\),

then what is \(\displaystyle x\)?

Possible Answers:

\(\displaystyle 8\)

\(\displaystyle 32\)

\(\displaystyle 128\)

\(\displaystyle 64\)

\(\displaystyle 16\)

Correct answer:

\(\displaystyle 32\)

Explanation:

This is a test of translating logarithmic/exponential properties, with the key here being to realize that

\(\displaystyle \log_{4}x = \frac{5}{2}\) is equivalent to \(\displaystyle x = 4^{5/2}\).

With that in mind, here is how it works out:

\(\displaystyle 4^{5/2} = (\sqrt[2]{4})^{5} = 2^{5} = 2*2*2*2*2 = 32\)

Hence, \(\displaystyle x = 32\).

Example Question #12 : Logarithms

\(\displaystyle \frac{\log{25}}{\log{5}}\)can be written as which of the following?

A. \(\displaystyle \frac{\log_{10}25}{\log_{10}5}\)

B. \(\displaystyle \log_{5}25\)

C. \(\displaystyle 2\)

Possible Answers:

A and B only

B only

B and C only

A only

A, B and C

Correct answer:

A, B and C

Explanation:

A is true in two ways. You can use the fact that if a logarithm has no base, you can use base 10, or you can use the fact that you can use this property:

\(\displaystyle \log_{a}b = \frac{\log_{c}b}{\log_{c}a}\)

B is a simple change of base application, and C is simply computing the logarithm.

\(\displaystyle \log_{5}25 = \frac{\log_{10}25}{\log_{10}5}\)

\(\displaystyle \log_5 25\rightarrow \log_ab=c \rightarrow a^c=b\)

\(\displaystyle \\ \log_5 25\\ 5^x=25\\5^2=25\\x=2\)

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