f(x)=cos2x/(sinx+cosx)
=(cos^2x-sin^2x)/(cosx+sinx)
=(cosx+sinx)(cosx-sinx)/(cosx+sinx)
=cosx-sinx
=√2cos(x+π/4)cosx-sinx为什么得到下面的不太懂cosx-sinx =√2(cosx*√2/2-sinx*√2/2) =√2(cosx*cosπ/4-sinx*sinπ/4) =√2cos(x+π/4)