sina+cosa=1/5,所以1+2sinacosa=1/25=1+sin2a,即sin2a=-24/25
tan(2a-π/4)=(tan2a-tanπ/4)/(1+tan2atanπ/4)=(tan2a-1)/(1+tan2a),因为2a属于（π,3π/2),且sin2a=-24/25,所以cos2a=-7/25,tan2a=24/7,所以tan(2a-π/4)=17/31