Diketahui cos A = 4/5 Sin B = 12/13. Tentukan : a.Sin (a+b) b.Cos 2a c.Sin 1/2a

sin A = 3/5 cos B = 5/13

a. sin (a+b)
    = sin a . cos b + cos a . sin b
    = 3/5 . 5/13 + 4/5 . 12/13
    = 9/65 + 48/65
    = 57/65

b. cos 2a
    = 1 - 2 . sin^2a
    = 1 - 2 . (3/5)^2
    = 1 - 2 . 9/25
    = 1 - 18/25
    = 7/25

c. sin 1/2a
    = akar 1-cos a / 2
    = akar 1 - 4/5 . 2
    = akar 1/5 / 2
    = akar 2/5