消掉t:
y=(5-t^2)/(1+t^2)=[6-(1+t^2)]/(1+t^2)=6/(1+t^2)-1
1/(1+t^2)=(y+1)/6
x=3t^2/(1+t^2)=[3(1+t^2)-3]/(1+t^2)=3-3/(1+t^2
x=3-3(y+1)/6
化简：y=-2x+5
表示的图形为直线