Question

What exactly deos the foci of an ellipse mean? What is its value in case of a circle and why?

Answer 1

Foci of the ellipse are the two point on the semi major axis, locus of all the point which have constant sum from foci, gives ellipse.

I the case of circle, there is only one point at the centre of the circle which is known as the focus or the centre of the circle.
Circle is the locus of all point which are at the same distance from the centre.