Given a general quadratic curve

the quantity X is known as the discriminant, where

and is invariant under rotation. Using the coefficients from quadratic equations for a rotation by an angle













Now let









and use






to rewrite the primed variables









From (11) and (13), it follows that

Combining with (12) yields, for an arbitrary









which is therefore invariant under rotation. This invariant therefore provides a useful shortcut to determining the shape represented by a quadratic curve.Choosing
to make
(see quadratic equation), the curve takes on the form

Completing the square and defining new variables gives

Without loss of generality, take the sign of H to be positive. The discriminant is

Now, if
, then
and
both have the same sign, and the equation has the general form of an ellipse (if
and
are positive). If
then
and
have opposite signs, and the equation has the general form of a hyperbola. If
, then either
or
is zero, and the equation has the general form of a parabola (if the nonzero
or
is positive). Since the discriminant is invariant, these conclusions will also hold for an arbitrary choice of
, so they also hold when
is replaced by the original
. The general result is
1. If
, the equation represents an ellipse, a circle (degenerate ellipse), a point (degenerate circle), or has no graph.
2.If
, the equation represents a hyperbola or pair of intersecting lines (degenerate hyperbola).
3. If
, the equation represents a parabola,a line (degenerate parabola), a pair of parallel lines (degenerate parabola), or has no graph.

(1)
the quantity X is known as the discriminant, where

(2)
and is invariant under rotation. Using the coefficients from quadratic equations for a rotation by an angle






(3)



(4)





(5)
Now let



(6)



(7)



(8)
and use



(9)



(10)
to rewrite the primed variables



(11)



(12)



(13)
From (11) and (13), it follows that

(14)
Combining with (12) yields, for an arbitrary










(15)
which is therefore invariant under rotation. This invariant therefore provides a useful shortcut to determining the shape represented by a quadratic curve.Choosing



(16)
Completing the square and defining new variables gives

(17)
Without loss of generality, take the sign of H to be positive. The discriminant is

(18)
Now, if
















1. If

2.If

3. If

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