Cos A.cos B=2/5 dan (A-B)=π/3 Maka cos(A+B)=...

Trigonometri

cos A cos B = 2/5
(A -B) = π/3 = 60°

cos (A - B) = sin A sin B + cos A cos B
cos 60 = sin A sin B + 2/5
1/2 = sin A sin B+ 2/5
sin A sin B  = 1/2 - 2/5
sin A sin B = 1/10

cos (A+B) = sin A sin B - cos A cos B
cos (A+b) = 1/10 - 2/5 = - 3/10