Let `aa n db` be nonzero real numbers. Then the equation `(a x^2+b y^2+c)(x^2-5x y+6y^2)=0` represents. four straight lines, when `c=0` and `a , b` are of the same sign. two straight lines and a circle, when `a=b` and `c` is of sign opposite to that `a` two straight lines and a hyperbola, when `aa n db` are of the same sign and `c` is of sign opposite to that of `a` a circle and an ellipse, when `aa n db` are of the same sign and `c` is of sign opposite to that of `a`