Sum Rule for Derivatives/Proof 2
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Theorem
Let $\map f x, \map j x, \map k x$ 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 $\map f x = \map j x + \map k x$.
Then $f$ is differentiable at $\xi$ and:
- $\map {f'} \xi = \map {j'} \xi + \map {k'} \xi$
It follows from the definition of derivative that if $j$ and $k$ are both differentiable on the interval $I$, then:
- $\forall x \in I: \map {f'} x = \map {j'} x + \map {k'} x$
Proof
It can be observed that this is an example of a Linear Combination of Derivatives with $\lambda = \mu = 1$.
$\blacksquare$