cara beserta penyelesaian

Dik : sin a = 3/5
sin b = 7/25
Dit : cos a cos b - sin a sin b?

cos a = akar 5^2-3^2
= akar 25-9
= akar 16
= 4

cos b = akar 25^2 -7^2
= akar 625 - 49
= akar 576
= 24
jdi
cos a cos b - sin a sin b
(4/5.24/25) - ( 3/5.7/25)
(20/25.24/25) -(15/25.7/25)
(480/25) - ( 105/25)
375/25 = 15