Logarithmic Functions

y = log a x if and only if x = a y The logarithmic function to the base a, where a > 0 and a  1 is defined: exponential form logarithmic form Convert to log form: Convert to exponential form: When you convert an exponential to log form, notice that the exponent in the exponential becomes what the log is equal to.

LOGS = EXPONENTS With this in mind, we can answer questions about the log: This is asking for an exponent. What exponent do you put on the base of 2 to get 16? (2 to the what is 16?) What exponent do you put on the base of 3 to get 1/9? (hint: think negative) What exponent do you put on the base of 4 to get 1? When working with logs, re-write any radicals as rational exponents. What exponent do you put on the base of 3 to get 3 to the 1/2? (hint: think rational)

In the last section we learned about the graphs of exponentials. Logs and exponentials are inverse functions of each other so let’s see what we can tell about the graphs of logs based on what we learned about the graphs of exponentials. Recall that for functions and their inverses, x’s and y’s trade places. So anything that was true about x’s or the domain of a function, will be true about y’s or the range of the inverse function and vice versa. Let’s look at the characteristics of the graphs of exponentials then and see what this tells us about the graphs of their inverse functions which are logarithms.

Characteristics about the Graph of an Exponential Function a > 1 1. Domain is all real numbers 2. Range is positive real numbers 3. There are no x intercepts because there is no x value that you can put in the function to make it = 0 4. The y intercept is always (0,1) because a 0 = 1 5. The graph is always increasing 6. The x-axis (where y = 0) is a horizontal asymptote for x  -  Characteristics about the Graph of a Log Function where a > 1 1. Range is all real numbers 2. Domain is positive real numbers 3. There are no y intercepts 4. The x intercept is always (1,0) (x’s and y’s trade places) 5. The graph is always increasing 6. The y-axis (where x = 0) is a vertical asymptote

Exponential Graph Logarithmic Graph Graphs of inverse functions are reflected about the line y = x

Transformation of functions apply to log functions just like they apply to all other functions so let’s try a couple. up 2 left 1 Reflect about x axis

The secret to solving log equations is to re-write the log equation in exponential form and then solve. Convert this to exponential form check: This is true since 2 3 = 8