Logarithms (or Logs)

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’. It follows that 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. $\log_a(x)$ is considered to be the inverse of [math]a^x[/math] – see Logarithmic Graphs.

Explore Example 1 to see more of this or Example 2 when a calculator is required to evaluate the logs.

Note that logging negative numbers with a positive base is not possible. This is because, there is no amount of times you can multiply a positive number by itself to get any negative number. Your calculator will verify this with a MATH ERROR.

Note that $\log_a(a)=1$ since $a^1=a$. Note also that log to the base is the natural logarithm and has the special notation ln. For example, $\log_e(8)=\ln(8)$. Interestingly,  logs used to be calculated on a slide rule – click here to find out more. You might also want to check out log graphs.

Log Rules

Arguably the most important identity when using logs is the following:

$\log_a(b)=c\hspace{10pt}\Longleftrightarrow\hspace{10pt}a^c=b$

This is because log equations (see below) can be solved once in this format.

The following rules follow from the Laws of Indices and should be used to convert log expressions into a single log.

$\begin{array}{c}\log_a(x)+\log_a(y)=\log_a(xy) \hspace{20pt}(1)\\\log_a(x)-\log_a(y)=\log_a(x/y)\hspace{20pt}(2)\\\log_a(x^n)=n\log_a(x)\hspace{20pt}(3)\end{array}$

You can also change the base of the log using the following formula:

$\log_a(x)=\frac{\log_b(x)}{\log_b(a)}$

See Example 8 to see an example of using a change of base.

The above log identities and rules can be used to solve log equations. Of course, if an equation is in the form $a^c=b$, logs can be applied to solve the equation. Additionally, if the equation involves an expression that is not yet in the format $\log_a{b}$, use the log rules to get it in this format. See Examples 5 to 8.

Examples of Logs

Evaluate

$\begin{array}{l}\log_3(9)\\
\log_4(64)\\
\log_2(1/8) \text{ since }2^{-3}\\\log_{123}(1)\end{array}
$

Evaluate the following:

$\begin{array}{l}\log_{9}(5)\\
\log_{8}(9)\\
\log_{12}(150)\\
\log_{2}(0.7)\\
\log_{3}(-4)\end{array}$

Write the following as a single logarithm:

$3\log_5(2)+2\log_5(4)-\log_5(6)$

Expand the following in terms of logs of x, y and z:

$\log_a\left(\frac{x^2\sqrt{y}}{z^5}\right)$

Find the exact solution of i) $4e^{2x-7}=20$ and ii) $\ln(3y-5)=6$.

Solve the equation $5^{x}+9\times 5^{-x}=6$.

Solve $2^xe^{3x}=e^5$.

Solve the equation   $\log_2(x)=\log_x(5)$.

Videos of Logs

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