Precalculus : Exponential and Logarithmic Functions

Study concepts, example questions & explanations for Precalculus

varsity tutors app store varsity tutors android store

Example Questions

Example Question #1 : Properties Of Logarithms

Find the inverse function of the following exponential function:

\displaystyle y = \frac{2}{3}e^{4x+1}

Possible Answers:

\displaystyle y = \frac{1}{4}\ln {\frac{3}{2}x}-\frac{1}{4}

\displaystyle y = \frac{1}{4}\ln {\frac{2}{3}x}-\frac{1}{4}

\displaystyle y = \frac{1}{4}\ln {\frac{3}{2}x}-1

\displaystyle y = \frac{1}{4}\ln {x}-\frac{1}{4}

\displaystyle y = \frac{3}{8}\ln {x}-\frac{1}{4}

Correct answer:

\displaystyle y = \frac{1}{4}\ln {\frac{3}{2}x}-\frac{1}{4}

Explanation:

Since we are looking for an inverse function, we start by swapping the x and y variables in our original equation.

\displaystyle x = \frac{2}{3}e^{4y+1}

Now we have to solve for y. To do this we have to work towards isolating y. First we remove the constant multiplier:

\displaystyle \frac{3}{2}x = \frac{3}{2}*\frac{2}{3}e^{4y+1}

\displaystyle \frac{3}{2}x = e^{4y+1}

Next we eliminate the base on the right side by taking the natural log of both sides.

\displaystyle \ln\frac{3}{2}x = \ln e^{4y+1}

\displaystyle \ln\frac{3}{2}x = {4y+1}

Subtract 1 and divide by 4:

\displaystyle \ln\frac{3}{2}x - 1 = {4y+1} - 1

\displaystyle \ln\frac{3}{2}x - 1 = 4y

\displaystyle \frac{1}{4}\left ( \ln\frac{3}{2}x - 1 \right ) = 4y*\frac{1}{4}

\displaystyle \frac{1}{4}\ln {\frac{3}{2}x}-\frac{1}{4} = y

\displaystyle y = \frac{1}{4}\ln {\frac{3}{2}x}-\frac{1}{4}

Example Question #11 : Properties Of Logarithms

Condense the following expression into one logarithm:

\displaystyle \ln{8x}+2\ln3x^2-\ln2

Possible Answers:

\displaystyle \ln36x^5

\displaystyle \ln12x^3

\displaystyle \ln36x^3

\displaystyle \ln12x^5

Correct answer:

\displaystyle \ln36x^5

Explanation:

To condense this, the second term must have the 2 in front moved inside.

\displaystyle \ln{8x}+\ln(3x^2)^2-\ln2

\displaystyle \ln8x+\ln9x^4-\ln2

When adding two logs, multiply their insides; when subtracting two logs, divide their insides.

\displaystyle \ln((8x)(9x^4))-\ln2

\displaystyle \ln72x^5-\ln2

\displaystyle \ln\frac{72x^5}{2}

\displaystyle \ln36x^5

Example Question #22 : Exponential And Logarithmic Functions

Expand the following log completely

\displaystyle \log_{3}(27x^4y^3)

Possible Answers:

\displaystyle 3-4\log_{3}x-3\log_{3}y

\displaystyle 3+4\log_{3}x+3\log_{3}y

\displaystyle \log_327+\log_3x+\log_3y

\displaystyle \log_327+4\log_3x+3\log_3y

\displaystyle \log_327+\log_3x^4+3\log_3y^3

Correct answer:

\displaystyle 3+4\log_{3}x+3\log_{3}y

Explanation:

To expand a logarithm, quantities in the inside that are multiplied get added and quantities in the inside that are divided get subtracted.

\displaystyle \log_{3}27+\log_3(x^4)+\log_3(y^3)

\displaystyle \log_33^3+4\log_3(x)+3\log_3(y)

\displaystyle 3+4\log_3(x)+3\log_3(y)

Example Question #22 : Exponential And Logarithmic Functions

Solve for \displaystyle x:

\displaystyle e^{2x}-5e^x+6=0

Possible Answers:

\displaystyle x=\ln3

\displaystyle x=\ln2

\displaystyle x=\ln6

\displaystyle x=\ln2

\displaystyle x=\ln3

\displaystyle x=\ln5

Correct answer:

\displaystyle x=\ln2

\displaystyle x=\ln3

Explanation:

The first step to solving this problem is to realize that

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

Then, the equation falls into the follow form which resembles a quadratic.

\displaystyle (e^x)^2-5e^x+6=0

Let \displaystyle y=e^x. Then,

\displaystyle y^2-5y+6=0

\displaystyle (y-3)(y-2)=0

Thus, \displaystyle y=2 and \displaystyle y=3.

Since \displaystyle y=e^x,

\displaystyle 2=e^x\Rightarrow \ln2=\ln{e^x}\Rightarrow x=\ln2

\displaystyle 3=e^x\Rightarrow \ln3=\ln{e^x}\Rightarrow x=\ln3

Example Question #11 : Properties Of Logarithms

Simplify the expression.

\displaystyle \log(72x)-\log(12)

Possible Answers:

None of the above answers

\displaystyle \log(7x)

\displaystyle \log(-6x)

\displaystyle \log(6x)

\displaystyle 6x

Correct answer:

\displaystyle \log(6x)

Explanation:

Using the quotient rule for logarithms we can condense these two logarithms into a single logarithm. 

\displaystyle \log\left(\frac{72x}{12}\right) 

We then obtain our answer by simple division.

Example Question #12 : Properties Of Logarithms

Simplify the expression.

\displaystyle -2\ln(xy^2)+3\ln(y)

Possible Answers:

\displaystyle \ln\left(\frac{1}{x^2y}\right)

\displaystyle -6\ln(xy^3)

\displaystyle \ln\left(\frac{y}{x^2}\right)

\displaystyle -6xy^3

\displaystyle \ln(x^2y)

Correct answer:

\displaystyle \ln\left(\frac{1}{x^2y}\right)

Explanation:

Using the properties of logarithms we first simplify the expression to \displaystyle \ln(y^3)-\ln(x^2y^4).  Then we use the quotient rule for logarithms and cancel some terms to obtain our answer.

Example Question #25 : Exponential And Logarithmic Functions

Simplify the expression.

\displaystyle 2\log(x)-3\log(y)+\log(x^2y^3)

Possible Answers:

None of the other answers

\displaystyle -6\log(x^4)

\displaystyle -6x^3y^4

\displaystyle \log(x^4)

\displaystyle \log(x^4y^6)

Correct answer:

\displaystyle \log(x^4)

Explanation:

Using the properties of logarithms we first rewrite the expression as \displaystyle \log(x^2)-\log(y^3)+\log(x^2y^3). Now we combine the three pieces into the form \displaystyle \log\left(\frac{x^2x^2y^3}{y^3}\right). We then obtain our answer when we combine the terms with\displaystyle x and cancel those with \displaystyle y.

Example Question #26 : Exponential And Logarithmic Functions

Solve for x:

\displaystyle 4^{x-2}=64

Possible Answers:

\displaystyle x=5

\displaystyle x=3

\displaystyle x=4

\displaystyle x=2

\displaystyle x=1

Correct answer:

\displaystyle x=5

Explanation:

\displaystyle 4^{x-2}=64

\displaystyle 4^{x-2}=4^3

\displaystyle \log_{4}4^{x-2}=\log_{4}4^3

\displaystyle x-2=3\Rightarrow x=5

Example Question #24 : Exponential And Logarithmic Functions

Which of the following is equivalent to \displaystyle (e^{3}\cdot e^{9})^{2} ?

Possible Answers:

\displaystyle e^{14}

\displaystyle e^{36}

\displaystyle e^{24}

\displaystyle e^{54}

\displaystyle e^{144}

Correct answer:

\displaystyle e^{24}

Explanation:

When multiplying exponents with a common base, you add both the exponents together. Hence, \displaystyle (e^{3}\cdot e^{9})=e^{12}

When an exponent is raised to an exponent you multiply the exponents together. Hence, for \displaystyle (e^{12})^{2} you would multiply \displaystyle (12\cdot 2) to get \displaystyle e^{24}.

Answer: \displaystyle e^{24}

Example Question #27 : Exponential And Logarithmic Functions

Solve the following for x:

\displaystyle 3^{2x+1}=81

Possible Answers:

\displaystyle \frac{5}{2}

\displaystyle \frac{3}{2}

\displaystyle 2

\displaystyle 0

\displaystyle 1

Correct answer:

\displaystyle \frac{3}{2}

Explanation:

\displaystyle 3^{2x+1}=81

\displaystyle 3^{2x+1}=3^4

\displaystyle \log_33^{2x+1}=\log_33^4

\displaystyle 2x+1=4

\displaystyle 2x=3

\displaystyle x=\frac{3}{2}

Learning Tools by Varsity Tutors