Algebra II : Solving and Graphing Logarithms

Study concepts, example questions & explanations for Algebra II

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Example Questions

Example Question #1 : Solving Logarithms

Solve for \(\displaystyle x\):

\(\displaystyle 5^{x} = \left(\frac{1}{25}\right)^{4x-1}\)

Possible Answers:

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

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

\(\displaystyle x = \frac{-2}{7}\)

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

\(\displaystyle x = \frac{2}{9}\)

Correct answer:

\(\displaystyle x = \frac{2}{9}\)

Explanation:

To solve for \(\displaystyle x\), first convert both sides to the same base:

\(\displaystyle 5^{x} = \frac{1}{25}^{4x-1} = (5^{-2})^{4x-1} = 5^{-8x+2}\)

Now, with the same base, the exponents can be set equal to each other:

\(\displaystyle x= - 8x +2\)

Solving for \(\displaystyle x\) gives:

\(\displaystyle x = \frac{2}{9}\)

Example Question #2 : Solving Logarithms

Solve the equation: 

\(\displaystyle \log{_{4}}{x^{2}}+\log{_{4}}{3}=1\)

Possible Answers:

\(\displaystyle \frac{\sqrt{6}}{3}, -\frac{\sqrt{6}}{3}\)

\(\displaystyle 1,-1\)

\(\displaystyle \frac{2\sqrt{3}}{3}, -\frac{2\sqrt{3}}{3}\)

\(\displaystyle \frac{\sqrt{3}}{3}, -\frac{\sqrt{3}}{3}\)

\(\displaystyle \frac{\sqrt{3}}{2}, -\frac{\sqrt{3}}{2}\)

Correct answer:

\(\displaystyle \frac{2\sqrt{3}}{3}, -\frac{2\sqrt{3}}{3}\)

Explanation:

\(\displaystyle \log{_{4}}{x^{2}}+\log{_{4}}{3}=1\)

\(\displaystyle \Rightarrow \log{_{4}}{x^{2}}+\log{_{4}}{3}=\log{_{4}}{4}\)

\(\displaystyle \Rightarrow log{_{4}}{(x^{2}\times 3)}=\log{_{4}}{4}\)

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

\(\displaystyle \Rightarrow x^{2}=\frac{4}{3}\)

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

Example Question #3 : Solving Logarithms

Use  \(\displaystyle \small \small \log_23\approx 1.5850\)  to approximate the value of  \(\displaystyle \small \log_2 24\).

Possible Answers:

\(\displaystyle \small \log_2 24\approx 3.4150\)

\(\displaystyle \small \log_2 24\approx 12.5850\)

\(\displaystyle \small \log_2 24\approx 3.5850\)

\(\displaystyle \small \log_2 24\approx 4.5850\)

\(\displaystyle \small \log_2 24\approx 12.6800\)

Correct answer:

\(\displaystyle \small \log_2 24\approx 4.5850\)

Explanation:

Rewrite \(\displaystyle 24\) as a product that includes the number \(\displaystyle 3\):

\(\displaystyle \small \log_2 24 = \log_2 (2^3 \cdot 3)\)

Then we can split up the logarithm using the Product Property of Logarithms:

\(\displaystyle \small \log_2 (2^3\cdot 3) = \log_2 2^3 + \log_2 3\)

                     \(\displaystyle \small =3+\log_2 3\)

                     \(\displaystyle \small \approx 3 + 1.5850\)

Thus,

\(\displaystyle \small \log_2 24 \approx 4.5850\).

 

Example Question #1 : Solving Logarithms

Solve for \(\displaystyle x\).

\(\displaystyle log_{9}^{ }(2x+1)=2\)

Possible Answers:

\(\displaystyle x=\frac{17}{2}\)

\(\displaystyle x=40\)

\(\displaystyle x=41\)

\(\displaystyle x=20\)

\(\displaystyle x=\frac{81}{2}\)

Correct answer:

\(\displaystyle x=40\)

Explanation:

Rewrite in exponential form:

\(\displaystyle 9^{2}=2x+1\)

Solve for x:

\(\displaystyle 81=2x+1\)

\(\displaystyle x=40\)

Example Question #2 : Solving Logarithms

Solve the following equation:

\(\displaystyle ln(3x+8)=0\)

Possible Answers:

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

\(\displaystyle -\frac{8}{3}\)

\(\displaystyle -\frac{7}{3}\)

\(\displaystyle 1\)

\(\displaystyle \frac{8}{7}\)

Correct answer:

\(\displaystyle -\frac{7}{3}\)

Explanation:

For this problem it is helpful to remember that,

 \(\displaystyle ln(3x+8) = 0\) is equivalent to \(\displaystyle ln(3x+8) = ln(1)\) because \(\displaystyle ln(1)=0\)

Therefore we can set what is inside of the parentheses equal to each other and solve for \(\displaystyle x\) as follows:

\(\displaystyle 3x+8=1\)

\(\displaystyle 3x=-7\)

\(\displaystyle x=-\frac{7}{3}\)

Example Question #5 : Solving Logarithms

Solve for \(\displaystyle x\):

\(\displaystyle log_6x=5\)

Possible Answers:

\(\displaystyle 15625\)

\(\displaystyle 1\)

\(\displaystyle 7776\)

\(\displaystyle 10000\)

\(\displaystyle 30\)

Correct answer:

\(\displaystyle 7776\)

Explanation:

To solve this logarithm, we need to know how to read a logarithm. A logarithm is the inverse of an exponential function. If a exponential equation is

\(\displaystyle a^c = b\)

then its inverse function, or logarithm, is

\(\displaystyle log_ab=c\)

Therefore, for this problem, in order to solve for \(\displaystyle x\), we simply need to solve

\(\displaystyle 6^5=x\)

which is \(\displaystyle 7776\).

Example Question #7 : Solving Logarithms

\(\displaystyle \log x+\log\left ( x+21\right )=2\)

Solve for \(\displaystyle x\).

Possible Answers:

\(\displaystyle -25\)

\(\displaystyle 4\)

\(\displaystyle -4\)

\(\displaystyle 25\)

\(\displaystyle 100\)

Correct answer:

\(\displaystyle 4\)

Explanation:

Logs are exponential functions using base 10 and a property is that you can combine added logs by multiplying.

\(\displaystyle x(x+21)=10^{2}\)

\(\displaystyle x^{2}+21x=100\)

\(\displaystyle (x+25)(x-4)=0\)

You cannot take the log of a negative number.  x=-25  is extraneous.

Example Question #1 : Solving Logarithms

If  \(\displaystyle \log_{x}36=2\), which of the following is a possible value for \(\displaystyle x\)?

Possible Answers:

\(\displaystyle 6\)

\(\displaystyle 4\)

\(\displaystyle 2\)

\(\displaystyle 18\)

\(\displaystyle 9\)

Correct answer:

\(\displaystyle 6\)

Explanation:

This question is testing the definition of logs. \(\displaystyle \log_{a}b=x\) is the same as \(\displaystyle a^{x}=b\).

In this case, \(\displaystyle \log_{x}36=2\) can be rewritten as \(\displaystyle x^{2}=36\).

Taking square roots of both sides, you get \(\displaystyle x=\pm 6\). Since only the positive answer is one of the answer choices, \(\displaystyle 6\) is the correct answer.

Example Question #5 : Solving Logarithms

Rewriting Logarithms in Exponential Form

Solve for \(\displaystyle x\) below: 

\(\displaystyle 3^{x}=15\)

Which of the below represents this function in log form? 

Possible Answers:

\(\displaystyle log{_{x}}{3}=15\)

\(\displaystyle log{_{15}}{3}=x\)

\(\displaystyle log{_{3}}{15}=x\)

\(\displaystyle log{_{x}}{15}=3\)

Correct answer:

\(\displaystyle log{_{3}}{15}=x\)

Explanation:

The first step is to rewrite this equation in log form. 

When rewriting an exponential function as a log we must remember that the form of an exponential is: 

\(\displaystyle b^{^{x}}=a\)

When this is rewritten in log form it is: 

\(\displaystyle log{_{b}}(a)=x\).

Therefore we have \(\displaystyle 3^{x}=15\) which when rewritten gives us,

\(\displaystyle log{_{3}}{15}=x\).

Example Question #10 : Solving Logarithms

Solve for \(\displaystyle x\):

\(\displaystyle log_3x=\frac{1}{3}log_364+\frac{1}{2}log_349\).

Possible Answers:

Not enough information

\(\displaystyle x=45.8\)

\(\displaystyle x=28\)

\(\displaystyle x=11\)

\(\displaystyle x=522.7\)

Correct answer:

\(\displaystyle x=28\)

Explanation:

Use the rule of Exponents of Logarithms to turn all the multipliers into exponents:

\(\displaystyle log_3x=log_364^\frac{1}{3}+log_349^\frac{1}{2}\).

Simplify by applying the exponents: \(\displaystyle log_3x=log_34+log_37\).

According to the law for adding logarithms, \(\displaystyle log_b(mn) = log_b(m) + log_b(n)\).

Therefore, multiply the 4 and 7.

\(\displaystyle log_3x=log_328\).

Because both sides have the same base, \(\displaystyle x=28\).

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