是不是表示一个圆?
[x-(t+3)]^2+[y+(1-4t^2)]^2=-16t^4-9+(t+3)^2+(1-4t^2)^2
r^2=-16t^4-9+(t+3)^2+(1-4t^2)^2
=-7t^2+6t+1
表示一个圆则r^2>0
-7t^2+6t+1>0
7t^2-6t-1