sinα+cosα=1/5
(sinα+cosα)^2=1+2sinα*cosα=(1/5)^2=1/25
sin2α=2sinα*cosα=-24/25
cos2α=-7/25
tan2α=sin2α/cos2α=24/7
tan（2α-π/4）=(tan2α-tanπ/4）/(1+tan2α*tanπ/4)=17/31