Question

已知sina=3/5,且a属于(π/2,π),则sin(π/4+a)

Answer 1

a属于(π/2,π),,所以cosa<0

sina=3/5 于是 cosa = -4/5

sin(π/4+a) =sinπ/4cosa + sinacosπ/4 = √2/2 * (-1/5) = -√2/10

Answer 2

因为sina=3/5,且a属于(π/2,π), 所以 cosa=-4/5
所以sin(π/4+a)=sinπ/4*cosa+cosπ/4*sina=负十分之根号二