,
finish it ...
At this point, I can't use L'Hopital's rule. I tried to solve this using two different approaches and continue to get a 0 in the denominator even though I know that 0 should cancel out somehow. Here's the exercise:
Use the definition of the derivative to show that d/dx[cos x] = -sin x.
Hint: Use the limit laws in section 1.6 and the identity cos (A + B) = cos A cos B - sin A sin B.
Please help. This is one of two problems that I have left to do and have spent lots of time trying to solve to no avail.
Sorry, but I don't know what to use to cancel out the h's in the denominators. I've gone through the entire section 1.6 and don't see anything that I believe I can use to address this. Should I be looking more closely at the trigonometric identities or properties to resolve this?