f（x）=sin（x+π/4）+2sin（x-π/4）-4cos2x+3cos（x+3π/4)
=√2/2sinx+√2/2cosx+√2sinx-√2cosx-3√2/2sinx+3√2/2cosx-4cos2x
=√2cosx-4cos2x
=-8cos^2x+√2cosx+4