f（x）=（1-tanx）[1+√2sin(2x+π/4) ]
=(1-tanx)(1+sin2x+cos2x)
=(1-tanx)(2sinxcosx+2cosxcosx)
=2(1-tanx)cosx(sinx+cosx)
=2(cosx-sinx)(cosx+sinx)
=2cos2x
定义域：(-pai/2+kpai,pai/2+kpai)(k属于Z)
周期T＝2pai/2=pai