if one of the circles x^2+y^2+2g1x+c=0 and x^2+y^2+2g2x+c=0 lies within the other then prove that g1g2>0 and c>0
So now assume that c>0. Check that in this case, neither circle contains any points on the y-axis. So each circle (and its interior) lies entirely on one side of the y-axis. Your next step is to find where the centres of the circles are. If they are on opposite sides of the y-axis then neither circle can lie inside the other. So the centres must lie on the same side of the y-axis. Deduce that and have the same sign and therefore .