Constant factor rule in differentiation

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In calculus, the constant factor rule in differentiation, also known as the Kutz Rule[citation needed], allows one to take constants outside a derivative and concentrate on differentiating the function of x itself. This is a part of the linearity of differentiation.

Consider a differentiable function

where k is a constant.

Use the formula for differentiation from first principles to obtain:

This is the statement of the constant factor rule in differentiation, in Lagrange's notation for differentiation.

In Leibniz's notation, this reads

If we put k=-1 in the constant factor rule for differentiation, we have:

Comment on proof[edit]

Note that for this statement to be true, k must be a constant, or else the k can't be taken outside the limit in the line marked (*).

If k depends on x, there is no reason to think k(x+h) = k(x). In that case the more complicated proof of the product rule applies.