f(x)=(cosx+sinx)/(cosx-sinx) 上下除cosx
=(1+tanx)/(1-tanx)
=(tanπ/4+tanx)/(1-tanπ/4*tanx)
=tan(x+π/4)是不是用到tan(α+β)的公式？是的tan(α+β)=(tanα+tanβ)/(1-tanαtanβ)tanπ/4=1