# Logarithmic Derivative of Product of Analytic Functions

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## Theorem

Let $D \subset \C$ be open.

Let $f, g: D \to \C$ be analytic.

Let $z \in D$ with $f \left({z}\right) \ne 0 \ne g \left({z}\right)$.

Then:

- $\dfrac{\left({f g}\right)' \left({z}\right)} {\left({f g}\right) \left({z}\right)} = \dfrac{f' \left({z}\right)} {f \left({z}\right)} + \dfrac {g' \left({z}\right)} {g \left({z}\right)}$

## Proof

Follows directly from Complex Derivative of Product.

$\blacksquare$