Question

I'm having trouble visualizing why in the geometric proof of the derivative of sine the two angles are congruent. Professor Jerison does something about rotating by 90 degrees the triangle but i quite don't understand why. This proof is given in lecture 3.

Answer 1

Yes, I find that "explanation" unsatisfying

we could make this argument:
where I used "a" rather than theta. and not a (a with bar) to represent 90-a