divf(x)=acosx(-sinx+cosx)-asinx(cosx+sinx)=a(cos^2x-sin^2x-2cosxsinx)
=a(cos2x-sin2x)≥0
=>2x∈[-3π/4+2kπ,π/4+2kπ],k∈Z
=>x∈[-3π/8+kπ,π/8+kπ],k∈Z
divf(x)=acosx(-sinx+cosx)-asinx(cosx+sinx)=a(cos^2x-sin^2x-2cosxsinx)
=a(cos2x-sin2x)≥0
=>2x∈[-3π/4+2kπ,π/4+2kπ],k∈Z
=>x∈[-3π/8+kπ,π/8+kπ],k∈Z