f(x)=sin(2x+pi/6)+sin(2x-pi/6)+2(cosx)^2+a
=2sin2xcos(pi/6)+cos2x+1+a
注意到sin(pi/6)=1/2
f(x)=2(sin2xcos(pi/6)+cos2xsin(pi/6))+1+a
=2sin(2x+pi/6)+1+a
-pi/4