Locus - cartesian equation of a circle

Consider a circle with centre (h,k) and radius r.

Let P (x,y) be any point on the circle.

Using the distance formula / Pythagoras' Theorem, show that the equation of the circle is:

                             (x - h)² + (y - k)² = r²

Explain the equation of a circle in terms of transformations of a circle with the same radius but centre the origin.