cos(a-pai/12)= - 4/5,=> sin(a-pai/12)=3/5,
cos(2a-pai/6)= 7/25, sin(2a-pai/6)= - 24/25
设 u=2a-pai/6
cos(2a+pai/12) =cos(u+pai/4)
=√2/2 (cosu - sinu)
= 31√2 / 50