Hey there MHF.
I got a quick question with hopefully a quick answer.
How does ?
Think of the unit circle. If you draw in a radius, it makes an angle with the positive x-axis. Then draw a segment from the end of the radius to the x-axis, you will have a right-angle triangle. The horizontal length is and the vertical length is . Since it's a right-angle-triangle, Pythagoras' Theorem holds, and so , or . This is true no matter the value of .