Exponentials and Logarithms

Logarithms are the Inverse of Exponentials

\log_{a}(b)=c means that a^{c}=b

a^{1}=a for any a so \log_{a}(a)=1

a^{0}=1 for any a so \log_{a}(1)=0

 

Example: \log_{3}(81)=4 because 3^{4}=81

 

We also have other rules:

\log_{a}(x)+\log_{a}(y)=\log_{a}(xy)

\log_{a}(x)-\log_{a}(y)=\log_{a}\left(\dfrac{x}{y}\right)

\log_{a}(x^{n})=n\log_{a}(x)

\log_{a}(x)=\dfrac{\log_{b}(x)}{\log_{b}(a)}

 

Note: The little number after the \text{log} is called the base. The most common base is 10, but this is usually left out – i.e. for \text{log}_{10} we just write \text{log} .

Using Logarithms and Exponentials to Solve Equations

We can use the rules above – called the laws of logarithms – to solve equations that involve exponentials.

 

Example: 3^{4x}=5

\log(3^{4x})=\log(5)

4x\log(3)=\log(5)

\begin{aligned}x&=\dfrac{\log(5)}{4\log(3)}\\[1.2em]&=0.366\end{aligned}

Note: The base of the logarithm was not given in this example. This is because we would get the same answer using any base.

 

On the flipside, we can use exponentials to solve equations involving logarithms.

Example: 6\log_{4}(x)=17

\log_{4}(x)=\dfrac{17}{6}

\begin{aligned}x&=4^{\frac{17}{6}}\\[1.2em]&=50.8\end{aligned}

Graphs of Exponentials

An exponential graph is a graph of the function y=a^{x} for some a>0. They all have the same basic shape. If a>1 then y increases as x increases. If a<1 then y decreases as x increases. Larger a gives faster increase if a>1 while smaller a gives faster decrease if a<1.

Note: Exponential graphs never reach \mathbf{0}.

On the left graph is y=2^{x}, y=3^{x} and y=4^{x}.

On the right graph is y=\left(\dfrac{1}{2}\right)^{x}, y=\left(\dfrac{1}{3}\right)^{x} and y=\left(\dfrac{1}{4}\right)^{x}.