Tentukan nilai cos4a jika diketahui sina+cosa =1/2

sina + cosa = 1/2
kuadratkan kedua ruas
(sina + cosa)^2 = (1/2)^2
(sina)^2 + (cosa)^2 + 2sina.cosa = 1/4
1 + sin2a = 1/4
sin2a = 1/4 - 1 = - 3/4

cos4a = 1 - 2(sin2a)^2
= 1 - 2(-3/4)^2
= 1 - 2(9/16)
= 1 - 9/8 = 1/8