f(x)=2(cosx)^3+(sinx)^2+cosx-3/2-2cosx+cosx
=2(cosx)^3+1-(cosx)^2-3/2
=2(cosx)^3-(cosx)^2-1/2
f(π/3)=2(1/2)^3-(1/2)^2-1/2
=-1/2