HELP!!! trigonometry 1. The problem statement, all variables and given/known data without using calculator: prove that: sin18 = √5-1/4 2. Relevant equations Double-angle formulae sin 2a = 2sin a.cos a cos 2a = cos^2a - sin^2a = 1 - 2sin^2a = 2cos^2a - 1 sin^2 a + cos^2 a=1 3. The attempt at a solution let 18=a sin a = sin(90 - a) = cos 4a cos 4a = 1 - 2sin^2 2a = 1 - 2(2sin a.cos a)^2 = 1 - 2(4sin^2 a.cos^2 a) = 1 - 8sin^2 a.cos^2 a = 1 - 8sin^2a(1-sin^2a) = 1 - 8sin^2a + 8sin^4a i can't go further than that ?????!!