Question

Let a, b be such that π < α – β < 3π. If sinα + sinβ = –21/65, and cosα + cosβ = –27/65, then the value of cos((α -β)/2) is 

(a) 6/65

(b) 3/√(130)

(c) - 3/√(130)

(d) - 6/65.

Answer 1

by squaring and adding we get