已知sina=3/4,a属于（派/2,派）,求sin(a+派/4),cos(a-派/3)的值.
问题描述：
已知sina=3/4,a属于（派/2,派）,求sin(a+派/4),cos(a-派/3)的值.
答
sina=3/4
cosa = -√7/4
sin(a+π/4)
= (√2/2)(sina+ cosa)
= (√2/8)(3-√7)
cos(a-π/3)
=(1/2)cosa + (√3/2)sina
= -√7/4 + 3√3/8