Find the derivative of f(x) = 9x + 5 at x = 7.

asked Apr 23, 2019 151k views

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$f(x)=9x+5\implies f'(x)=9\implies f'(7)=9$

Or, using the definition of the derivative,

$f'(7)=\displaystyle\lim_(x\to7)(f(x)-f(7))/(x-7)$

$\displaystyle=\lim_(x\to7)((9x+5)-(63+5))/(x-7)$

$\displaystyle=\lim_(x\to7)(9(x-7))/(x-7)$

$\displaystyle=\lim_(x\to7)9=9$

Croigsalvador answered Apr 28, 2019

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