Editing List of logarithmic identities (section)

==== Logarithm of a quotient ====

To state the ''logarithm of a quotient'' law formally:

:<math>\forall b \in \mathbb{R}_+, b \neq 1, \forall x, y, \in \mathbb{R}_+, \log_b \left( \frac{x}{y} \right) = \log_b(x) - \log_b(y)</math>

Derivation:

Let <math>b \in \mathbb{R}_+</math>, where <math>b \neq 1</math>,
and let <math>x, y \in \mathbb{R}_+</math>.

We want to relate the expressions <math>\log_b(x)</math> and <math>\log_b(y)</math>. This can be done more easily by rewriting in terms of exponentials, whose properties we already know. Additionally, since we are going to refer to <math>\log_b(x)</math> and <math>\log_b(y)</math> quite often, we will give them some variable names to make working with them easier: Let <math>m = \log_b(x)</math>, and let <math>n = \log_b(y)</math>.

Rewriting these as exponentials, we see that:
:<math>\begin{align}
m &= \log_b(x) \iff b^m = x, \\
n &= \log_b(y) \iff b^n = y.
\end{align}</math>

From here, we can relate <math>b^m</math> (i.e. <math>x</math>) and <math>b^n</math> (i.e. <math>y</math>) using exponent laws as

:<math>\frac{x}{y} = \frac{(b^m)}{(b^n)} = \frac{b^m}{b^n} = b^{m - n}</math>

To recover the logarithms, we apply <math>\log_b</math> to both sides of the equality.

:<math>\log_b \left( \frac{x}{y} \right) = \log_b \left( b^{m -n} \right)</math>

The right side may be simplified using one of the logarithm properties from before: we know that <math>\log_b(b^{m - n}) = m - n</math>, giving

:<math>\log_b \left( \frac{x}{y} \right) = m -n</math>

We now resubstitute the values for <math>m</math> and <math>n</math> into our equation, so our final expression is only in terms of <math>x</math>, <math>y</math>, and <math>b</math>.

:<math>\log_b \left( \frac{x}{y} \right) = \log_b(x) - \log_b(y)</math>

This completes the derivation.