Precalculus : Exponential and Logarithmic Functions

Study concepts, example questions & explanations for Precalculus

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Example Questions

Example Question #31 : Exponential And Logarithmic Functions

Solve for x:

\(\displaystyle 2^x=128\)

Possible Answers:

\(\displaystyle 3\)

\(\displaystyle 7\)

\(\displaystyle 5\)

\(\displaystyle 4\)

\(\displaystyle 6\)

Correct answer:

\(\displaystyle 7\)

Explanation:

\(\displaystyle 2^x=128\)

\(\displaystyle 2^x=2^7\)

\(\displaystyle \log_22^x=\log_22^7\)

\(\displaystyle x=7\)

Example Question #21 : Properties Of Logarithms

Solve the equation:

\(\displaystyle 2^x4^2=8^{^{3+x}}\)

Possible Answers:

\(\displaystyle x=\frac{2}{5}\)

\(\displaystyle x=\frac{5}{2}\)

\(\displaystyle x=-2\)

\(\displaystyle x=\frac{-2}{5}\)

\(\displaystyle x=\frac{-5}{2}\)

Correct answer:

\(\displaystyle x=\frac{-5}{2}\)

Explanation:

We notice that all of the bases are powers of 2 so we can re-write them as 2 raised to a certain power. 

\(\displaystyle 2^x(2^2)^2=(2^3)^{3+x}\)

Then we simplify both sides of the equation so we have a single base 2 raised to a power. When we have when we have exponentials with the same base multiplied we add the exponents, when we have them raised to an exponent we multiply the exponents. 

\(\displaystyle 2^{x+4}=2^{9+3x}\)

Now that we have the same base on both sides we set the exponents equal and solve for x.

\(\displaystyle x+4=9+3x\)

\(\displaystyle 2x=-5\)

\(\displaystyle x=\frac{-5}{2}\)

Example Question #22 : Properties Of Logarithms

The logarithms for this question are common logs (i.e. base 10).

Solve for \(\displaystyle x\) in the following expression.  Round to three decimal places.

\(\displaystyle \log(x^2+10x+25) +\log (3x+15)=4 + \log 3\)

Possible Answers:

\(\displaystyle 17.021\)

\(\displaystyle 16.544\)

\(\displaystyle 41.416\)

\(\displaystyle 21.544\)

Correct answer:

\(\displaystyle 16.544\)

Explanation:

The expression can be simplified using logarithmic properties.

\(\displaystyle \log(x^2+10x+25) +\log (3x+15)=4 + \log 3 \\\)

\(\displaystyle \log(x+5)^2 +\log (3*(x+5))=4 + \log 3\)

\(\displaystyle 2\log(x+5) +\log 3 + \log(x+5)= 4 + \log 3\)

\(\displaystyle 3 \log(x+5)=4\)

We use the definition of common logarithm to solve for x.

\(\displaystyle (x+5)^3=10^4\)

\(\displaystyle (x+5)=\sqrt[3]{10000}=21.544\)

So x = 21.544 -5 or 16.544.

Example Question #23 : Properties Of Logarithms

Solve for the value of x by simplifying the following logarithmic function:

\(\displaystyle log (x)+log(x+3) =1\)

Possible Answers:

\(\displaystyle x=-5\)

\(\displaystyle x=2\)

\(\displaystyle x=3\)

\(\displaystyle x=5\)

\(\displaystyle x=-2\)

Correct answer:

\(\displaystyle x=5\)

Explanation:

Log Property: Log(x) + Log(y) = Log(xy)

The equation \(\displaystyle log (x)+log(x+3) =1\) can be simplified using the log property stated above. 

The simplified version of the equation is:

\(\displaystyle log((x)(x-3))=1\)

Substitute Log(10) in for 1, since Log(10) = 1

\(\displaystyle log((x)(x-3))=log(10)\)

Next, remove the logs from both sides

\(\displaystyle (x)(x-3)=10\)

Solve for the value of x:

\(\displaystyle x(x-3)=10\)

\(\displaystyle x^{2}-3x=10\)

\(\displaystyle x^{2}-3x-10=0\)

\(\displaystyle (x-5)(x+2)=0\)

\(\displaystyle x=5, x=-2\)

A negative number cannot be evaluated in the log function equation above so the answer is \(\displaystyle x=5\)

Example Question #24 : Properties Of Logarithms

Find the value of x to satisfy \(\displaystyle log{_{16}}{4^{x}}=2\).

Possible Answers:

\(\displaystyle x=4\)

\(\displaystyle x=12\)

\(\displaystyle x=16\)

\(\displaystyle x=2\)

\(\displaystyle x=3\)

Correct answer:

\(\displaystyle x=4\)

Explanation:

First, remove the log from the equation by changing the form:

\(\displaystyle log{_{16}}{4^{x}}=2\)

\(\displaystyle 16^{2}=4^{x}\)

Next, change both sides so that they share a common base. In this case the common base is 4.

\(\displaystyle (4^{2})^{2}=4^{x}\)

Reduce the amount of exponents on the left side. Since an exponent is raised to an exponent you multiply the exponents together.

\(\displaystyle 4^{4}=4^{x}\)

Set the exponents on both sides equal to eachother and solve for x.

\(\displaystyle 4=x\)

 

Answer: \(\displaystyle x=4\)

Example Question #25 : Properties Of Logarithms

Simplify 

\(\displaystyle x=\log_{16}\left(\frac{1}{64}\right)\).

Possible Answers:

\(\displaystyle x= \frac{-1}{4}\)

\(\displaystyle x= \frac{1}{4}\)

\(\displaystyle x=-\frac{3}{2}\)

\(\displaystyle x=4\)

\(\displaystyle x=-4\)

Correct answer:

\(\displaystyle x=-\frac{3}{2}\)

Explanation:

Since the logarithmic function \(\displaystyle y=\log_{b}(x)\) is the inverse of the exponential function \(\displaystyle x=b^y\), we can make a straightforward simplification as follows:

\(\displaystyle \frac{1}{64}=16^x\)

\(\displaystyle 64^-^1=16^x\)

\(\displaystyle 2^-^6=2^{4x}\)

\(\displaystyle -6 \log2=4x \log2\)

\(\displaystyle -6=4x\)

\(\displaystyle x=-\frac{3}{2}\)

Example Question #26 : Properties Of Logarithms

Solve the equation,

 \(\displaystyle x=\log_{8}2+\log_{8}4+\log_{8}8\).

Possible Answers:

\(\displaystyle 4\)

\(\displaystyle 16\)

\(\displaystyle 1\)

\(\displaystyle 2\)

\(\displaystyle 8\)

Correct answer:

\(\displaystyle 2\)

Explanation:

Using the logarithmic product rule \(\displaystyle \log_b(xy)= \log_bx + \log_b y\), we simplify as follows:

\(\displaystyle \log_{8}2+\log_{8}4+\log_{8}8 = \log_8(8\cdot4\cdot2)\)

\(\displaystyle \log_864\)

\(\displaystyle 8^x=64\)

\(\displaystyle x=2\)

Example Question #27 : Properties Of Logarithms

Simplify the expression \(\displaystyle \sqrt{\log_4{16x}}\).

Possible Answers:

\(\displaystyle 2+\frac{1}{2}log_4 x\)

\(\displaystyle 1+\frac{1}{2}log x\)

\(\displaystyle 1+\frac{1}{2}log_4 x\)

\(\displaystyle 1\frac{1}{2}log_4 x\)

\(\displaystyle 2+\log_4 x\)

Correct answer:

\(\displaystyle 1+\frac{1}{2}log_4 x\)

Explanation:

Utilizing the power rule of logs we reduce the original expression to:

 \(\displaystyle \frac{1}{2}{}\log_4{16x}\)

From there we use the product property to obtain:

 \(\displaystyle \frac{1}{2}(\log_416+log_4 x)\)

Since 4 to the second power is 16 this expression yields 

\(\displaystyle \frac{1}{2}(2+log_4 x)\) , then we multiply through by the one half to obtain our final answer \(\displaystyle \left(1+\frac{1}{2}log_4 x\right)\).

Example Question #28 : Properties Of Logarithms

\(\displaystyle \left(log_a \left(\frac{b}{cd}\right)\right)^e\) is the simplified form of which of the following expressions?

Possible Answers:

\(\displaystyle e(log_a d-log_a c-log_a b)\)

\(\displaystyle (log_a b-log_a c-log_a d)^e\)

\(\displaystyle e(log_a c-log_a b-log_a d)\)

\(\displaystyle e(log_a b-log_a c-log_a d)\)

\(\displaystyle e(log_a d-log_a b-log_a c)\)

Correct answer:

\(\displaystyle e(log_a b-log_a c-log_a d)\)

Explanation:

First use the power property of logs to get:

 \(\displaystyle e log_a\frac{b}{cd}\).

Then use the quotient property to get:

 \(\displaystyle e (log_a b -log_acd)\).

Then finally use the product property and simplify to yield the final answer, \(\displaystyle e (log_a b -log_ac-log_ad)\).

Example Question #29 : Properties Of Logarithms

Simplify. 

\(\displaystyle \frac{8^49^3}{2^{10}3^4}\)

Possible Answers:

\(\displaystyle 2^{2}3^2\)

\(\displaystyle 32\)

\(\displaystyle \frac{18}{5}\)

\(\displaystyle 36\)

\(\displaystyle \frac{864}{240}\)

Correct answer:

\(\displaystyle 36\)

Explanation:

\(\displaystyle \frac{8^49^3}{2^{10}3^4}\) 

We observe that \(\displaystyle 8=2^3\) and \(\displaystyle 9=3^2\) and substitute to yeild

\(\displaystyle \frac{(2^3)^4(3^2)^3}{2^{10}3^4}\)

Now we use the fact that \(\displaystyle (a^b)^c=a^{b*c}\) to simplify  

\(\displaystyle \frac{2^{12}3^6}{2^{10}3^4}\)

Since \(\displaystyle \frac{a^b}{a^c}=a^{b-c}\) simplifying again yeilds

\(\displaystyle 2^23^2\) which is just \(\displaystyle 4\cdot9\)

This yeilds a final answer of 36

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