a-π/6
0＜a-π/6＜π/2
cos(a-π/6)=4/5
sin(a-π/6)=3/5;
sina=sin(a-π/6)cosπ/6+cos(a-π/6)sinπ/6
=3/5 *√3/2 + 4/5 *1/2
= (3√3 + 4)/10