α是第三象限角sinα<0 cosα<0 tanα>0cosα^2=1-sinα^2=1-25/169=144/169cosα=-12/13tanα=sinα/cosα=-5/13/(-12/13)=5/12
α是第三象限角，Cosα < 0 和tanα > 0cosa = -根号(1 - (5/13)^2) = - 12/13tana = sina /cosa = 5 / 12
