cos(α-π/3)=12/13
则,sin(α-π/3)=5/13
cosα
=cos[(α-π/3)+π/3]
=cos(α-π/3)cosπ/3-sin(α-π/3)sinπ/3
=12/13×1/2-5/13×√3/2
=(12-5√3)/26