Precalculus : Logarithmic Functions

Study concepts, example questions & explanations for Precalculus

varsity tutors app store varsity tutors android store

Example Questions

Example Question #1 : Exponential And Logarithmic Functions

You are given that \(\displaystyle \log_{10} x = A\) and \(\displaystyle \log_{10} y = B\)

Which of the following is equal to \(\displaystyle 100^{2A-B}\) ?

Possible Answers:

\(\displaystyle \frac{2x}{y}\)

\(\displaystyle \frac{x^{4}}{y^{2}}\)

\(\displaystyle \frac{y^{2}}{x^{4}}\)

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

\(\displaystyle \frac{x}{\sqrt{y}}\)

Correct answer:

\(\displaystyle \frac{x^{4}}{y^{2}}\)

Explanation:

Since \(\displaystyle \log_{10} x = A\) and \(\displaystyle \log_{10} y = B\), it follows that \(\displaystyle 10^{A} = x\) and \(\displaystyle 10^{B} = y\)

\(\displaystyle 100^{2A-B} = \left( 10^{2} \right ) ^{2A-B} =10^{2 (2A-B)} =10^{4A-2B}=\frac{10^{4A}}{10^{2B}} =\frac{\left ( 10^{A}\right )^4}{\left (10^{B}\right )^2}=\frac{x^4}{y^2}\)

Example Question #2 : Logarithmic Functions

Solve.

\(\displaystyle \ln(x^2-4)+\ln(3)=0\)

Possible Answers:

\(\displaystyle x=4\)

\(\displaystyle x=\pm\sqrt{\frac{13}{3}\)

\(\displaystyle x=\pm2\)

None of the other answers

\(\displaystyle x=\pm\sqrt{\frac{4}{3}}\)

Correct answer:

\(\displaystyle x=\pm\sqrt{\frac{13}{3}\)

Explanation:

First we condense the LHS to \(\displaystyle \ln(3x^2-12)\) using the properties of logarithms.  Then we can eliminate the natural log on the LHS by raising both sides of the equation as exponents with base \(\displaystyle e\). This leaves us with \(\displaystyle 3x^2-12=1\) since \(\displaystyle e^0 = 1\).  Now we do some simple Algebra and obtain our answer.

Example Question #3 : Logarithmic Functions

Solve.

\(\displaystyle 10^{2x}=64\)

Possible Answers:

\(\displaystyle \log(8)\)

\(\displaystyle 2\log(8)\)

\(\displaystyle \log(64)\)

\(\displaystyle \ln(8)\)

None of the other answers

Correct answer:

\(\displaystyle \log(8)\)

Explanation:

First take the logarithm of both sides.  This yields \(\displaystyle \log10^{2x}=\log(64)\). Now using the properties of logarithms, we can bring the \(\displaystyle 2x\) down in front of \(\displaystyle \log(10)\). Since \(\displaystyle \log(10)=1\). We are left with \(\displaystyle 2x = \log(64)\).  Dividing each side by \(\displaystyle 2\) we find \(\displaystyle x = \frac{1}{2}\log(64) = \log(\sqrt{64})=\log(8)\).

Example Question #1 : Logarithmic Functions

Evalute the equation \(\displaystyle e^2^x^+^1=100\). Use a calculator to approximate the answer to three decimal places.

Possible Answers:

\(\displaystyle 0.500\)

\(\displaystyle 4.605\)

\(\displaystyle 2.281\)

\(\displaystyle 1.021\)

\(\displaystyle 1.803\)

Correct answer:

\(\displaystyle 1.803\)

Explanation:

First, we rewrite the equation in logarithmic form to obtain \(\displaystyle 2x+1=\ln100\). Simplifying, we get

\(\displaystyle 2x = ln100 - 1\)

\(\displaystyle x=\frac{ln100-1}{2}\)

\(\displaystyle x\approx1.803\)

Example Question #4 : Logarithmic Functions

Simplify

\(\displaystyle \log_464\)

Possible Answers:

\(\displaystyle 64\)

\(\displaystyle 3\)

\(\displaystyle 4\)

\(\displaystyle 0\)

\(\displaystyle 2\)

Correct answer:

\(\displaystyle 3\)

Explanation:

\(\displaystyle \log_464\rightarrow \log_44^3\rightarrow 3\)

Example Question #2 : Exponential And Logarithmic Functions

Solve the following equation for \(\displaystyle x\):

\(\displaystyle 4^x-16^{x-2}=0\)

Possible Answers:

\(\displaystyle 2\)

\(\displaystyle 16\)

\(\displaystyle -2\)

\(\displaystyle -4\)

\(\displaystyle 4\)

Correct answer:

\(\displaystyle 4\)

Explanation:

Logs must be used to solve this problem. Follow the steps below:

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

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

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

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

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

\(\displaystyle -x=-4\)

\(\displaystyle x=4\)

Example Question #6 : Logarithmic Functions

Solve.

\(\displaystyle \log_{2}(p-7)+\log_{2}(p)=3\)

Possible Answers:

\(\displaystyle p=8\)

\(\displaystyle p=-8,-1\)

\(\displaystyle p=7,2\)

\(\displaystyle p=-8,1\)

\(\displaystyle p=7,-2\)

Correct answer:

\(\displaystyle p=8\)

Explanation:

First we condense the LHS to \(\displaystyle \log_{2}(p^2-7p)\). Now we can eliminate a log of base \(\displaystyle 2\) by making both sides of the equation exponents of base \(\displaystyle 2\). This leaves us with \(\displaystyle p^2-7p=8\). Moving \(\displaystyle 8\) to the LHS we obtain a simple quadratic which we solve to obtain \(\displaystyle 8\) and \(\displaystyle -1\) for solutions. Finally, we eliminate \(\displaystyle -1\) as logarithms are undefined for negative numbers.

Example Question #2 : Logarithmic Functions

Find the domain of the function \(\displaystyle f(x)=\)  \(\displaystyle \log_{4}(8 - 4x)\).

Possible Answers:

\(\displaystyle [0,\infty )\)

\(\displaystyle (-\infty ,\infty )\)

\(\displaystyle (0,2)\)

\(\displaystyle (2,\infty )\)

\(\displaystyle (-\infty,2)\)

Correct answer:

\(\displaystyle (-\infty,2)\)

Explanation:

The logarithmic function is undefined when the inputs are negative or 0. Therefore the inputs of the logarithmic function must be positive. This means that the quantity \(\displaystyle 8-4x\) must be positive. After setting up the appropriate inequality, we have,

\(\displaystyle 8-4x>0\)

\(\displaystyle -4x>-8\)

\(\displaystyle x< 2\)

Therefore the domain of the function \(\displaystyle f(x)=\)  \(\displaystyle \log_{4}(8 - 4x)\) is the interval \(\displaystyle (-\infty,2)\).

Example Question #3 : Exponential And Logarithmic Functions

Use the logarithmic base change formula to convert \(\displaystyle \log_38\) to a quotient of logarithms with a base of \(\displaystyle 4\).

Possible Answers:

\(\displaystyle \log_38=\log_4\frac{3}{8}\)

\(\displaystyle \log_38=\frac{\log_48}{\log_43}\)

\(\displaystyle \log_38=\frac{\log_43}{\log_48}\)

\(\displaystyle \log_38=\log_4\frac{8}{3}\)

\(\displaystyle \log_38=\log_43\)

Correct answer:

\(\displaystyle \log_38=\frac{\log_48}{\log_43}\)

Explanation:

Here we employ the use of the logarithm base change formula 

\(\displaystyle \log_bx= \frac{\log_cx}{\log_cb}\).

Therefore we get,

\(\displaystyle \log_38=\frac{\log_48}{\log_43}\)

Example Question #4 : Logarithmic Functions

Find the domain of the function \(\displaystyle f(x)=\log_{10}(1+\sqrt{-6-3x})\).

Possible Answers:

\(\displaystyle (2,\infty)\)

\(\displaystyle (-2,\infty)\)

\(\displaystyle (-\infty,-2]\)

\(\displaystyle (-\infty,2]\)

\(\displaystyle (-\infty,-2)\)

Correct answer:

\(\displaystyle (-\infty,-2]\)

Explanation:

The logarithmic function is undefined when the inputs are negative or 0. Therefore the inputs of the logarithmic function must be positive. This means that the quantity \(\displaystyle 1+\sqrt{-6-3x\) must be positive. However, this quantity is always positive because the value under the radical cannot be negative. So we are only constrained by:

\(\displaystyle \sqrt{-6-3x} \geq 0\)

\(\displaystyle -6-3x \geq0\)

\(\displaystyle -3x\geq6\)

\(\displaystyle x\leq-2\)

Therefore the domain of the function \(\displaystyle f(x)=\log_{10}(1+\sqrt{-6-3x})\) is the interval \(\displaystyle (-\infty,-2]\).

Learning Tools by Varsity Tutors