f(x)=sinxcosπ/6-cosxsinπ/6+sinxcosπ/6+cosxsinπ/6+cosx+a
=2sinxcosπ/6+cosx+a
=√3sinx+cosx+a
=2sin(x+π/6)+a
f(x)=2sin(x+π/6)+a
-π/2