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[tex]Ax^2+Bxy+Cy^2+Dx+Ey+F=0 [/tex]

is

(a) elipse when ##B^2-4AC<0##

(b) parabola when ##B^2-4AC=0##

(c) hyperbola when ##B^2-4AC>0##

I found this in Thomas Calculus. However for some values of parameters ##A=17##, ##C=8##, ##B=\sqrt{4 \cdot 17 \cdot 8}##, ##D=E=0##, ##F=20## I got just straight line. Where is mistake?