Evaluate log base 4 of 21

$\log_{4} (21)$

The change of base rule can be used if $a$ and $b$ are greater than $0$ and not equal to $1$ , and $x$ is greater than $0$ .

$\log_{a} (x) = \frac{\log_{b} (x)}{\log_{b} (a)}$

Substitute in values for the variables in the change of base formula, using $b = 10$ .

$\frac{\log (21)}{\log (4)}$

The result can be shown in multiple forms.

Exact Form:

$\frac{\log (21)}{\log (4)}$

Decimal Form:

$2.19615871 \dots$