Diketahui tan c = p, c di kuadran 1. cos 2C

dibawa ke segitiga siku siku tan c = p/1 =depan/samping
nilai sisi miring = akar (p^2+1)

cos 2c = cos^2c - sin^2c
= 1/p^2+1 - p^2/p^2+1
=1 -p^2 / 1+ p^2