已知cosa=-5/13,a∈(π/2,π),则sin(a-π/3)=
问题描述：
已知cosa=-5/13,a∈(π/2,π),则sin(a-π/3)=
答
sina=√(1-cos²a)=12/13
sin(a-π/3)=sinacosπ/3-cosasinπ/3
=12/13×1/2-(-5/13)×√3/2
=6/13+5√3/26