# Logarithms

Logarithms, or logs for short, are essentially powers and are useful when a power is unknown.

When you come across logs, you will usually see the word ‘log’ followed by a small subscript then a number in brackets: $\log_a(b)=c.$

The subscript is known as the base and the number in brackets (although sometimes the brackets are left out) is the exponent.

It can help to understand logs by making a habit of, when reading log expressions, saying ‘the power of’ instead of the word ‘log’. The above reads as ‘the power of a to get a result of b is c’. For example, in $\log_2(8)$ the base is 2 and since it can be read as the power of 2 to give 8, the value of $\log_2(8)$ is 3.
Some other examples include: $\log_3(9)=2$ $\log_4(64)=3$ $\log_2(1/8)=-3$ since $2^{-3}=\frac{1}{2^3}=\frac{1}{8}$. $\log_{123}(1)=0$ since anything to the power of 0 is 1.

The equation $\log_a(b)=c$ can be used interchangeably with the equation $a^c=b$.

E.g. $\log_5(25)=x$ can be changed to $5^x=25$ and so x is 2.

There is a button on your calculator that can help you with powers that are not calculable in your head. The button has the word log followed by a two boxes; insert your base into the small lower box and the number that you see in brackets in the second. Use it to verify the following:

a) $\log_{9}(5)=0.732$ to 3 decimal places.

b) $\log_{8}(9)=1.06$ to 2 decimal places.

c) $\log_{12}(150)=2.016$ to 3 decimal places.

d) $\log_{2}(0.7)=-0.515$ to 3 decimal places.

e) $\log_{3}(-4)=N/A$

Note that the final example has no solution – it is not possible to determine the power of 3 that gives a result of -4. There is no amount of times you can multiply 3 by itself to get any negative number. Your calculator will verify this with a MATH ERROR.

See more on logs: