How to get the discriminant for symbolic quadractice equation?

I got a quadractic equation save in f.

How can i calculate d= b^2-4ac from that equation?

[Chunru]

Include code instead of screen capture.

Answer 1

[Torsten]

I guess you mean d = b^2-4*a*c if the polynomial is given by p(x)=a*x^2+b*x+c. So the a and b in the discriminant have nothing to do with your symbolic variables a and b, do they ?

syms a b x y x2 y2
f = x2/a^2 + y2/b^2;
f1 = x2 + y2 - 2*y;
x2_sol = solve(f==1,x2);
f1 = subs(f1,x2,x2_sol);
f1 = subs(f1,y2,y^2);
c = coeffs(f1,y);
d = c(2)^2-4*c(1)*c(3)

d =