_{n+1}= x

_{n}+ c

_{1}and y

_{n+1}= y

_{n}+ c

_{2}where c1 and c2 are constant values.

For a circle, they aren't constants - they need to be recalculated for each step:

x

_{n}= r * cos(a) and y

_{n}= r * sin(a) where r is the radius and a in the angle in radians. For each step, you change a slightly (how much depends on you and how accurate you need your circle: the bigger the increment, the less circular and the more polygonal it looks). Try 0.1, and check it's value against pi * 2 to see if you have reached the end of the circle.