diketahui cos (A-B) = 3/5 dan tan A . tan B = 2. ditanya cos (A+B)?

tan a tan b = 2
(sin a sin b)/(cos a cos b) = 2
sin a sin b = 2(cos a cos b)

cos (a-b) = cos a cos b + sin a sin b
3/5 = cos a cos b + sin a sin b
3/5 = cos a cos b + 2 cos a cos b
3/5 = 3 cos a cos b
3/(5×3) = cos a cos b
cos a cos b = 3/15

cos (a+b) = cos a cos b - sin a sin b
= cos a cos b - 2(cos a cos b)
= -1(cos a cos b)
= -1(3/15)
= -3/15