Jika sin teta-2 cos teta=0,maka cos 2teta

sinx - 2cosx =0
sinx = 2cosx
sinx/cosx = 2
tan x = 2
tan x = 2/1

sin x = 2/akar5

cos2x = 1 - 2.sin^2 x
= 1 - 2.(2/akar5)^2
= 1 - 8/5
= -3/5