tanx=sinx/cosx
又tanx=2 ==> cosx不等于0
（sinx-cosx）/ (sinx+cosx)上下同除于cosx
得[sinx/cosx -cosx/cosx] / [sinx/cosx + cosx/cosx]
=(tanx-1)/(tanx+1)
=(2-1)/(2+1)
=1/3