# Suppose f is differentiable at a.

• Nov 12th 2006, 08:30 PM
Swamifez
Suppose f is differentiable at a.
I am still contemplating over this proble, so if anyone is out there who can help me thanks. Suppose that the function f is diﬀerentiable at a. Prove (without
quoting a theorem) that f 2 is diﬀerentiable at a.
• Nov 12th 2006, 08:38 PM
Soltras
Quote:

Originally Posted by Swamifez
I am still contemplating over this proble, so if anyone is out there who can help me thanks. Suppose that the function f is diﬀerentiable at a. Prove (without
quoting a theorem) that f 2 is diﬀerentiable at a.

What do you mean by f 2?
• Nov 12th 2006, 08:50 PM
Swamifez
f squared is what i meant.
• Nov 12th 2006, 10:36 PM
srinivas
f is differentiable at a implies that f'(a) = [f(a+h) - f(a)]/h as h->0
take g(x) = [f(x)]^2
therefore lim h->0 [[f(a+h)]^2 - [f(a)]^2 ]/h = f'(a) * [f(a+h)+f(a)]
and u can easily see that the second bracket is equal to 2*f(a) as h->0
so the square of f is differentiable at a and the derivative is 2f(a)*f'(a)