Sum Rule for Derivatives

Theorem
Let $f \left({x}\right), j \left({x}\right), k \left({x}\right)$ be real functions defined on the open interval $I$.

Let $\xi \in I$ be a point in $I$ at which both $j$ and $k$ are differentiable.

Let $f \left({x}\right) = j \left({x}\right) + k \left({x}\right)$.

Then $f$ is differentiable at $\xi$ and:
 * $f' \left({\xi}\right) = j' \left({\xi}\right) + k' \left({\xi}\right)$

It follows from the definition of derivative that if $j$ and $k$ are both differentiable on the interval $I$, then:


 * $\forall x \in I: f' \left({x}\right) = j' \left({x}\right) + k' \left({x}\right)$