Prove that

sin² x + cos² x = 1

The unit circle is x² + y² = 1
cos(θ) = x coordinate of (1,0) under a rotation θ and
sin(θ) = y coordinate of (1,0) under a rotation θ by definition.

Substitute them in:
cos²(θ) + sin²(θ) = 1

Posted by Jer on 2012-12-21 17:45:47