Why Doesn't My 2-Sphere Graph Correctly in GraphCalc?

[yourdadonapogostick]

i attempted to graph a 2-sphere on graphcalc and it wouldn't work. the equations had to be in terms of z, so i had to use: $$z=\sqrt{-x^2-y^2+9}$$ and $$z=-\sqrt{-x^2-y^2+9}$$. i thought that would give me a 2-sphere at the origin with a radius of 3, but it didn't. i got:[PLAIN]http://photobucket.com/albums/a192/YoDizzle/th_almostsphere.bmp.[/URL] what did i do wrong?

the split is along the x y plane. if it matters, this is my first try at using graphcalc.

 

[rachmaninoff]

You got a sphere. It just breaks down near the xy-plane, because the approximations get fuzzy as the slope becomes very large. Your program only evaluates the function value on a small grid of points.