如果角a的顶点在原点,且另一边为x轴的话,tan a = -5/12
sin a = -12/13,cos a = 5/13
sin π/6 = 1/3,cos π/6 = √3/2
sin(a+π/6)=(-12/13)(√3/2)+(1/3)(5/13)
=-12√3/26+5/39
=5/39-12√3/26
=10/78-36√3/78
=(10-36√3)/78
=(5-18√3)/39