The circle

[Freijo]

I'm learning to programm with the SVGAlib, because graphics are more
enjoyable. But I get surprised yesterday when I tried to draw a circle from
the equation x^2+y^2=radius^2 ( y= sqrt{radius^2-x^2} ) but it doesn't draws
a circle, but a curved line.
What I'm doing bad?
Sorry if I'm annoying you.