因为π/3〈α〈π/2
所以 0〈α-π/3〈π/26
所以sin(α-π/3）=5/13
故cosα=cos(α-π/3+π/3）=cos(α-π/3）*cos(π/3)-sin(α-π/3)*sin(π/3)=6/13-（根号3）*6/13