The formula for a circle is `(x-h)^2+(y-k)^2=r^2` where the center of the circle is at (h,k) and the radius is r.
The formula for an ellipse is `((x-h)^2)/a^2+((y-k)^2)/b^2=1` where (h.k) is the center of the ellipse and a and b are the semi-major and simi-minor axes.
Assume we have an ellipse centered at (h,k) whose major and minor axis have equal length. This describes a circle centered at (h,k).
`(x-h)^2+(y-k)^2=a^2` by multiplying through by `a^2`
which is the formula for a circle centered at (h,k). Thus the formula for a circle is a special case of the formula for an ellipse.