Question

question about the proof of trig addition formulas for cos:

Answer 1

sorry about the scrolling: http://en.wikipedia.org/wiki/Proofs_of_trigonometric_identities

on the part with the diagram for trig addition formulas, where does "RPQ = (PI/2) - RPQ" come from?

Answer 2

also any sites with a better, easy to understand, definition would be greatly appriciated

Answer 3

RPQ = pi/2 - RQP

pi/2 radians is 90 degrees.

RPQ = 90 degrees - RQP

which we can see because that triangle is a right triangle, so the other two angles have to be complementary to add up to 180 degrees interior.

Answer 4

but PRQ is only one of the complementary angles, how do i know that it is half of 90?

Answer 5

I believe PRQ is meant to be a right angle, 90 degrees or pi/2 radians, although it doesn't appear to be indicated as such...

Answer 6

Just drawing diagram here