sin2x/cos(x+π/4)=sin2x/(cosxcosπ/4-sinxsinπ/4)=sin2x/[√2/2*(cosx-sinx)]=sin2x/(√2/2*4/5)=sin2x/(2√2/5)cosx-sinx=4/5两边平方cos²x+2sinxcosx+sin²x=16/251+sin2x=16/25sin2x=-7/25f(cosx)=co...