Since you wrote b≥a, and not b>a, If b=a, then a=b, so the formula still applies, but it's not an oval, it's a circle. Technically an elipse has to have the semi-major axis (a) longer than the semi-minor axis (b).

But there is a detailed explanation of your question here:

https://en.wikipedia.org/wiki/Semi-major_and_semi-minor_axes