Diketahui cos x = 4 / 5 dengan 0 < x < 90° . Tentukanlah nilai : sin 3x + sin x

cosx = 4/5
maka
sinx = 3/5

sin3x + sinx
= sin(2x + x) + sinx
= sin2x . cosx + cos2x . sinx + sinx
= (2 sinx . cosx) cosx + sinx (cos2x + 1)
= (2 sinx . cos²x) + 2 cos²x . sinx
= 2 sinx . cosx (cosx + cosx)
= 2(3/5)(4/5) (4/5 + 4/5)
= 24/25 (8/25)
= 216/625