tan(a+π/4)=(tana+tanπ/4)/(1-tanatanπ/4)=1/77tana+7=1-tanatana=sina/cosa=-3/4sina=-3cosa/4sin²a=9cos²a/16sin²a+cos²a=1所以cos²a=16/25,sin²a=9/25a在第二象限则cosa=-4/5,sina=3/5所以原式=-1/5
