The general equation of high school mathematics circle x ^ 2 + y ^ 2 + DX + ey + F = 0, f must be less than zero? What?

The general equation of high school mathematics circle x ^ 2 + y ^ 2 + DX + ey + F = 0, f must be less than zero? What?

It's not necessarily related to de. x ^ 2 + y ^ 2 + DX + ey + F = 0
(x+D/2)^2 +(y+E/2)^2 = -F +D/4 + E/4
-F +D/4 + E/4 = r^2 >0