How many points lie on the graph of $x^2+y^2=0$x2+y2=0?

How many points lie on the graph of $x^2+y^2=-81$x2+y2=−81?

State the equation of the circle.

A circle has its centre at the origin and a radius of $4$4 units.

Develop the equation for a circle with centre (0, 0) and radius r, by applying the formula for the length of a line segment;