Algebra II : Logarithms

Study concepts, example questions & explanations for Algebra II

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Example Questions

Example Question #3091 : Algebra Ii

Solve \(\displaystyle log_2 (x^2 + 7) = 5\)

Possible Answers:

\(\displaystyle 3\)

\(\displaystyle 5, -5\)

\(\displaystyle -7\)

\(\displaystyle 3, -7\)

\(\displaystyle 4, -6\)

Correct answer:

\(\displaystyle 5, -5\)

Explanation:

First, let's change the equation to exponent form:

\(\displaystyle x^2+7=2^5\)

Then simplify:

\(\displaystyle x^2+7=32\)

And solve:

\(\displaystyle x^2=25\)

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

Both answers are valid because \(\displaystyle x\) in the original equation is squared, so any negative numbers don't cause the logarithm to become negative.

Example Question #41 : Solving And Graphing Logarithms

Solve \(\displaystyle log_6(4x)=log_6(x+9)\)

Possible Answers:

\(\displaystyle 6\)

\(\displaystyle 0\)

\(\displaystyle 1\)

\(\displaystyle 3\)

\(\displaystyle 7\)

Correct answer:

\(\displaystyle 3\)

Explanation:

We can start by getting both the log terms on the same side of the equation:

\(\displaystyle log_6(4x)-log_6(x+9)=0\)

Then we combine log terms:

\(\displaystyle log_6(\frac{4x}{x+9})=0\,\)

Now we can change to exponent form:

\(\displaystyle \frac{4x}{x+9}=6^0\)

Anything raised to the \(\displaystyle 0\)th power equals \(\displaystyle 1\), and from here it becomes a simpler problem to solve:

\(\displaystyle \frac{4x}{x+9}=1\)

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

\(\displaystyle 3x=9\)

\(\displaystyle x=3\)

Example Question #3091 : Algebra Ii

Solve \(\displaystyle ln(x-2) + ln(x) = ln(2x+9)\)

Possible Answers:

\(\displaystyle -3\)

\(\displaystyle 3\)

\(\displaystyle 5\)

\(\displaystyle 1\)

\(\displaystyle e\)

Correct answer:

\(\displaystyle 3\)

Explanation:

First we're going to get all the natural logs on one side of the equation:

\(\displaystyle ln(x-2) + ln(x) - ln(2x+9)= 0\)

Next, we're going to combine all the terms into one natural log:

\(\displaystyle ln(\frac{(x-2)(x)}{2x+9}) = 0\)

Now we can change to exponent form:

\(\displaystyle \frac{(x-2)(x)}{2x+9} = e^0\)

Anything raised to the \(\displaystyle 0\)th power equals \(\displaystyle 1\), which helps us simplify:

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

\(\displaystyle (x-2)(x) = 2x+9\)

\(\displaystyle x^2-2x = 2x+9\)

\(\displaystyle x^2-9=0\)

From here, we can factor and solve:

\(\displaystyle (x+3)(x-3)=0\)

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

We have to notice, however, that \(\displaystyle -3\) isn't a valid answer because if we were to plug it into the original formula we would have a negative value in a logarithm. 

Example Question #3097 : Algebra Ii

Solve \(\displaystyle log (3 x + 1) - 2 log (x) = 1\).

Possible Answers:

\(\displaystyle -\frac{1}{5}\)

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

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

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

\(\displaystyle \frac{1}{10}\)

Correct answer:

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

Explanation:

The first thing we can do is write the coefficients in front of the logs as exponents of the terms inside the logs:

\(\displaystyle log (3 x + 1) - log (x^2) = 1\)

Next, we combine the logs:

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

Now we can change the equation into exponent form:

\(\displaystyle \frac{(3x+1)}{x^2}=10^1\)

When we simplify, we'll also move all the terms to one side of the equation:

\(\displaystyle 10x^2-3x-1=0\)

From here, we factor to get our solutions:

\(\displaystyle (5x+1)(2x-1)=0\)

\(\displaystyle x=\frac{1}{2}, -\frac{1}{5}\)

The last thing we have to do is check our answers.  \(\displaystyle \frac{1}{2}\) doesn't raise any problems, but \(\displaystyle -\frac{1}{5}\) does.  If we plug it back into the original equation, we would be evaluating a negative value in a log, which we can't do.

Example Question #51 : Solving Logarithms

Solve:  \(\displaystyle 2log_{2}(64)-3log_{4}(64)\)

Possible Answers:

\(\displaystyle 14\)

\(\displaystyle -3\)

\(\displaystyle 3\)

\(\displaystyle 7\)

\(\displaystyle 9\)

Correct answer:

\(\displaystyle 3\)

Explanation:

In order to solve for the logs, we will need to write the log properties as follows:

\(\displaystyle x^y = z\) and \(\displaystyle log_{x}z = y\)

This means that:

\(\displaystyle log_{2}(64) = 6\)

\(\displaystyle log_{4}(64) = 3\)

Replace the values into the expression.

\(\displaystyle 2log_{2}(64)-3log_{4}(64)= 2(6)-3(3) = 12-9 =3\)

The answer is:  \(\displaystyle 3\)

Example Question #3099 : Algebra Ii

Solve \(\displaystyle ln (e^{5x}) = 2\)

Possible Answers:

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

\(\displaystyle e\)

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

\(\displaystyle 1\)

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

Correct answer:

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

Explanation:

First, we rewrite the equation as:

\(\displaystyle log_e(e^{5x})=2\)

We can use log properties to simplify:

\(\displaystyle 5x=2\)

From here, simple algebra is used to solve:

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

Example Question #52 : Solving And Graphing Logarithms

Solve \(\displaystyle 2 log_x(64)=3\).

Possible Answers:

\(\displaystyle 8\)

\(\displaystyle 64\)

\(\displaystyle 32\)

\(\displaystyle 16\)

\(\displaystyle 4\)

Correct answer:

\(\displaystyle 16\)

Explanation:

First we divide both sides of the equation by \(\displaystyle 2\):

\(\displaystyle log_x(64)=\frac{3}{2}\)

Next, we write the equation in exponential form:

\(\displaystyle x^{\frac{3}{2}}=64\)

Then we solve for \(\displaystyle x\) by taking the cube root, and then squaring \(\displaystyle 64\):

\(\displaystyle x=16\)

Example Question #53 : Solving And Graphing Logarithms

Solve \(\displaystyle 2 log_{(x+1)}(9)=4\).

Possible Answers:

\(\displaystyle 1\)

\(\displaystyle .1\)

\(\displaystyle 2\)

\(\displaystyle 9\)

\(\displaystyle 3\)

Correct answer:

\(\displaystyle 2\)

Explanation:

First we can divide each side by \(\displaystyle 2\):

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

Next, we rewrite in exponent form:

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

From here, we simply take the square root of \(\displaystyle 9\), and then subtract \(\displaystyle 1\):

\(\displaystyle x=2\)

Example Question #52 : Solving Logarithms

Solve \(\displaystyle log_x(16)=4\)

Possible Answers:

\(\displaystyle 10\)

\(\displaystyle e\)

\(\displaystyle 1\)

\(\displaystyle .4\)

\(\displaystyle 2\)

Correct answer:

\(\displaystyle 2\)

Explanation:

First we rewrite the equation in exponential form:

\(\displaystyle x^4=16\)

Now we take the cube root of \(\displaystyle 16\):

\(\displaystyle x=2\)

Example Question #53 : Solving Logarithms

Solve \(\displaystyle 3+3log(2x)=12\).

Possible Answers:

\(\displaystyle 500\)

\(\displaystyle .5\)

\(\displaystyle 50\)

\(\displaystyle 5\)

\(\displaystyle 5000\)

Correct answer:

\(\displaystyle 500\)

Explanation:

First we subtract \(\displaystyle 3\) from both sides:

\(\displaystyle 3log(2x)=9\)

Then we divide both sides by \(\displaystyle 3\):

\(\displaystyle log(2x)=3\)

Now it would help if we wrote the equation in exponential form (remember, if the log doesn't show a base, it's base 10):

\(\displaystyle 2x=10^3\)

Finally, we use algebra to solve:

\(\displaystyle 2x=1000\)

\(\displaystyle x=500\)

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