Posted by **help needed** on Friday, July 6, 2012 at 3:17pm.

f(x)=1/x^2

use formal definition of derivative to find slope at x=2

- calculus -
**Steve**, Friday, July 6, 2012 at 3:48pm
f'(x) = lim(h->0) [1/(x+h)^2 - 1/x^2]/h

ignoring the limit notation for now, we have

(x^2 - (x+h)^2)/(hx^2(x+h)^2)

= (-2hx-h^2)/(hx^2(x+h)^2)

= -2(x+h)/(x^2(x+h)^2)

= -2/(x^2(x+h))

lim(h->0) is thus -2/x^3

at x=2, that's f'(2) = -1/4

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