Running into a problem with a TI-89 Program.

epicwinning97

Okay so recently I downloaded a Conic section program on my TI-89 Titanium. It has had several issues, which I managed to fix, but I can't fix this one. It can only sometimes find the radius of a circle. From the standard form equation 3x^2-12x+3y^2+6y-15=0 it gets the radius right, which is √10.

Now, on 2x^2-8x+2y^2+16y-4=0 it gets the radius wrong, it returns 2√17, instead of √22

From Ax^2+Bxy+C^2+Dx+Ey+F=0, where A=a, B=b, etc, their code for the radius is (-f/a+1/2*d)^2+(1/2*e)^2=r^2, which i assume they got from completing the square. Is there something wrong with this formula?