itu apaaaa yaaa tolong dong

cosacosb = 2/5
cos(a-b) = cosacosb + sinasinb
cos π/3 = 2/5 + sinasinb
½ = 2/5 + sinasinb
sinasinb = 1/10

cos(a+b) = cosacosb - sinasinb
= 2/5 - 1/10
= 3/10

jawaban: D

semoga membantu :)