(tana-2)/(3tana+5)=-5
tana-2=-15tana-25
16tana=-23
tana=-23/16
(sina)^2+3sinacosa-1
=[3sinacosa-(cosa)^2]/[(sina)^2+(cosa)^2]
=[3tana-1]/[(tana)^2+1]
=(-69/16-1)/(529/256+1)
=-(85/16)/(785/256)
=-1360/785
=-272/1573sinα+4cosα呢？tana=-23/16 如果a在第二象限，sina=23/根号785，cosa=-16/根号785 3sina+4cosa=5/根号785 如果a在第四象限，sina=-23/根号785，cosa=16/根号785 3sina+4cosa=-5/根号785