We all know circle as a very simple shape. But, it is actually so complicated. Even if it has no sides,it has a radius which defines the size of a circle. In order to identify the equation of a circle, you just need to know the center and the value of the radius and write it as (x-h)²+(y-k)²=r² where (h,k) is the center and r is the radius. Move the slider and see what happens.