Evaluate log base 5 of 7*3

$\log_{5} (7 \cdot 3)$

Multiply $7$ by $3$ .

$\log_{5} (21)$

Rewrite $\log_{5} (21)$ using the change of base formula.

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 $b = 10$ .

$\frac{\log (21)}{\log (5)}$

The result can be shown in multiple forms.

Exact Form:

$\frac{\log (21)}{\log (5)}$

Decimal Form:

$1.89166814 \dots$