由题意知：
cos(a-60°)=12/13
sin(a-60°)=5/13
cos60°=1/2
sin60°=√3/2
cosa=cos(a-60°+60°)
=cos(a-60°)cos60°-sin(a-60°)sin60°
=12/13 *1/2-5/13*√3/2
=(12-5√3)/26