"X squared + y squared = r squared" is the formula also known as the definition of a circle, where r represents the radius. If the formula was "x squared + y squared = 4," then the circle would have a radius of 2 because 2 squared equals 4.
A circle is a set of all points on a plane that are all a fixed distance from the center. If the circle was plotted on graph paper, the center would be located at (a,b) and the location of one point of the circumference is (x,y). It is possible to make a right-angled triangle with these two co-ordinates and a third unspecified one. Using Pythagoras' theorem, "a squared + b squared = c squared," the standard form for the equation is "(x-a) squared + (y-b) squared = r squared."