Sum rule in differentiation
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In calculus, the sum rule in differentiation is a method of finding the derivative of a function that is the sum of two other functions for which derivatives exist. This is a part of the linearity of differentiation. The sum rule in integration follows from it. The rule itself is a direct consequence of differentiation from first principles.
The sum rule states that for two functions u and v:
This rule also applies to subtraction and to additions and subtractions of more than two functions
Proof
Let h(x) = f(x) + g(x), and suppose that f and g are each differentiable at x. Applying the definition of the derivative and properties of limits gives the following proof that h is differentiable at x and that its derivative is given by h′(x) = f′(x) + g′(x).
A similar argument shows the analogous result for differences of functions. Likewise, one can either use induction or adapt this argument to prove the analogous result for a finite sum of functions. However, the sum rule does not in general extend to infinite sums of functions unless one assumes something like uniform convergence of the sum.^{[citation needed]}
References
 Gilbert Strang: Calculus. SIAM 1991, ISBN 0961408820, p. 71 (restricted online version (google books))
 sum rule at PlanetMath