Proof of the Sum-Difference Formulas - Part 1

In this video, I demonstrate how to prove the following sum-difference formulas, or trigonometric identities: cos(a - b) = cos(a)*cos(b) + sin(a)*sin(b) cos(a + b) = cos(a)*cos(b) - sin(a)*sin(b) sin(a + b) = sin(a)*cos(b) + sin(b)*cos(a) sin(a - b) = sin(a)*cos(b) - sin(b)*cos(a) From the results above, I also derive the following identities: cos(2a) = cos^2(a) - sin^2(a) sin(2a) = 2*sin(a)*cos(a) Thanks for watching. Please give me a "thumbs up" if you have found this video helpful. Please ask me a maths question by commenting below and I will try to help you in future videos. Follow me on Twitter! twitter.com/MasterWuMath cos(x - y) = cos(x)*cos(y) + sin(x)*sin(y) cos(x + y) = cos(x)*cos(y) - sin(x)*sin(y) sin(x + y) = sin(x)*cos(y) + sin(x)*cos(y) sin(x - y) = sin(x)*cos(y) - sin(x)*cos(y) cos(2x) = cos^2(x) - sin^2(x) sin(2x) = 2*sin(x)*cos(x)