b=(cosy,siny)=(-cos(x+π/6),-sin(x+π/6))
a(a+b) = a^2+ab = 1-[cosxcos(x+π/6)+sinxsin(x+π/6)] ,整理的（2-&3）/2（以下用&代表根号,懒得找了）
|a|=1 |a+b|=[(&3+2)cosx/2-sinx/2]^2 + [(&3+2)sinx/2+cosx/2]^2 = 2+&3
所以夹角为arccos(7-4&3)/2