Log functions are inverses of exponential functions. This means the domain of one is the range of the other. This is extremely helpful when solving an equation and the unknown is in an exponent. Before solving equations, you must be able to simplify expressions containing logs. The rules of exponents are applied, but in nonobvious ways. In order to get a conceptual handle on the properties of logs, it may be helpful to continually ask, what does a log expression represent? For example, what does
Guidance
Exponential and logarithmic expressions have the same 3 components. They are each written in a different way so that a different variable is isolated. The following two equations are equivalent to one another.
The exponential equation is read “
The two most common bases for logs are 10 and
There are four basic properties of logs that correlate to properties of exponents.
Addition/Multiplication:
Subtraction/Division:
Exponentiation:
There are a few standard results that should be memorized and should serve as baseline reference tools.

logb1=0 
logbb=1 
logb(bx)=x 
blogbx=x
Example A
Simplify the following expressions:

log464 
log1232 
log335 
log2128
Solution:

log464=log443=3⋅log44=3⋅1=3 
log1232=x can be rewritten as (12)x2−xx=32.=32=−5 
log335=5⋅log33=5 
log2128=log227=7
Example B
Write the expression as a logarithm of a single argument.
Solution: Note that the center expression is of a different base. First change it to base 2 by switching back to exponential form.
Thus the expression with the same base is:
Example C
Simplify the following expression:
Solution:
Concept Problem Revisited
A log expression represents an exponent. The expression
Vocabulary
A logarithm is a way of rewriting exponential equations to isolate the exponent.
Guided Practice
1. Prove the following log identity:
2. Rewrite the following expression under a single log.
3. True or false:
Answers:
1. Start by letting the left side of the equation be equal to
Therefore,
2.
3. Note, it may be very tempting to make errors in this practice problem.
False. It is true that the log of a product is the sum of logs. It is not true that the product of logs is the log of a sum.
Practice
Decide whether each of the following statements are true or false. Explain.
1.
2.
3.
Rewrite each of the following expressions under a single log and simplify.
4.
5.
6.
7.
Expand the expression as much as possible.
8. \begin{align*}\log_4 \left(\frac{2x^3}{5}\right)\end{align*}
9. \begin{align*}\ln \left(\frac{4xy^2}{15}\right)\end{align*}
10. \begin{align*}\log \left(\frac{x^2 (yz)^3}{3}\right)\end{align*}
Translate from exponential form to logarithmic form.
11. \begin{align*}2^{x+1}+4=14\end{align*}
Translate from logarithmic form to exponential form.
12. \begin{align*}\log_2 (x1)=12\end{align*}
Prove the following properties of logarithms.
13. \begin{align*}\log_{b^n}x=\frac{1}{n} \log_b x\end{align*}
14. \begin{align*}\log_{b^n}x^n=\log_b x\end{align*}
15. \begin{align*}\log_{\frac{1}{b}}\frac{1}{x}=\log_b x\end{align*}