Question

help?

Answer 1

What?

Answer 2

Ok, here's the explanation (by the way the answer is the third one down): Suppose theta determines a point T on the unit circle, with coordinates (x, y). By Pythagorean theorem, x^2 + y^2 = 1. Since x = cos theta and y = sin theta, we obtain the following identity:\[\sin ^{2}\theta+\cos ^{2}\theta=1\]
Here's a pic.