ab=2/5
所以(cos2α,sinα)*(1,2sinα－1)=cos2a+2(sina)^2-sina=2/5
1-2(sina)^2+2(sina)^2-sina=2/5
1-sina=2/5
sina=3/5
因为α∈(π/4,π),
所以当α∈(π/4,π/2)时,√2/2√2/2>3/5
所以a为钝角
cosa=-4/5
tana=-3/4
tan(α+π/4）=(tana+tanπ/4)/(1-tanatanπ/4)=(-3/4+1)/(1+3/4)=(1/4)/(7/4)=1/7