Example What is the radius of the circle given by the equation x 2 + y 2 = 15 The value of r 2 = 15 so the radius of the circle is 15 = 3.872983346 This answer can be left as a surd to give an ...

This means that, using Pythagoras’ theorem, the equation of a circle with radius r and centre (0, 0) is given by the formula x 2 + y 2 = r 2.