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?yàn)棣痢?π/4,π),
所以當(dāng)α∈(π/4,π/2)時(shí),√2/2
所以a為鈍角
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