The equation of a circle can be formed easily if we recall
the Pythagoras' theorem. again. A circle with centre at point $C=(c_1,c_2)$ and radius $r>0$ is the set of all points $(x,y)$ whose distance from $C$ is equal to $r$.
Hence

where $a$ and $b$ are positive parameters and $A = (c_1,c_2)$ is the centre of the ellipse. The parameters $a$ and $b$ define the length of the semi-major axis and the semi-minor axis. If $a=b$ then we get a circle.
A typical ellipse is depicted in Figure 5.4.