a属于(0,派)且cos(a-派/6)=3/5
所以
sin(a-派/6)=4/5
从而
cosa=cos（a-π/6＋π/6）
=cos（a-π/6）cosπ/6-sin（a-π/6）sinπ/6
=3/5×√3/2-4/5×1/2
=（3√3-4）/10