cos(a-30°)=4/5 ,0°≤a-30°≤60°
所以sin(a-30°)=3/5
所以cosa = cos(a-30°+30°)
=cos(a-30°)cos30°-sin(a-30°)sin30°
=4/5 * √3/2 - 3/5 * 1/2
=（4√3 -3）/10