x2−4x+y2=0x^{2}-4 x+y^{2}=0x2−4x+y2=0
x2−4x+y2+2y=0x^{2}-4 x+y^{2}+2 y=0x2−4x+y2+2y=0
x2+y2−4y=0x^{2}+y^{2}-4 y=0x2+y2−4y=0
x2+y2−4x=3x^{2}+y^{2}-4 x=3x2+y2−4x=3