Find Y at X on the circumference of a circle, centered at (0,0)

We have the circle centered at
O(0,0)

r is the
radius.

==> x^2 + y^2 =
r^2

(x,y) is a point on the
corcumference)

There are unlimited points on the
cicumference of the circle.

However, the relation between x
and y should be constant and defines with the equation x^2 +y^2 =
r^2

Points on the circumference could
be:

(r,0) (0, r) ( -r,0) (0,
-r)

Also, y= sqrt(r^2 - x^2)