sin（a＋派/3）+sina＝-(4√3)/5 ∴sinacosπ/3+cosasinπ/3+sina=-4√3/5 ∴3/2*sina+√3/2*cosa=-4√3/5 两边同时除以√3 ∴√3/2sina+1/2*cosa=-4/5 ∴sin(a+π/6)=-4/5 ∵-π/2
