Now we have come full circle. How do we identify the type of conic described by an equation? What happens when the axes are rotated? Recall, the general form of a conic is
If we apply the rotation formulas to this equation we get the form
It may be shown that . The expression does not vary after rotation, so we call the expression invariant. The discriminant, , is invariant and remains unchanged after rotation. Because the discriminant remains unchanged, observing the discriminant enables us to identify the conic section.
A General Note: Using the Discriminant to Identify a Conic
If the equation is transformed by rotating axes into the equation , then .
The equation is an ellipse, a parabola, or a hyperbola, or a degenerate case of one of these.
If the discriminant, , is
- , the conic section is an ellipse
- , the conic section is a parabola
- , the conic section is a hyperbola
Example 5: Identifying the Conic without Rotating Axes
Identify the conic for each of the following without rotating axes.
Solution
- Let’s begin by determining , and .
Now, we find the discriminant.
Therefore, represents an ellipse.
- Again, let’s begin by determining , and .
Now, we find the discriminant.
Therefore, represents an ellipse.
Candela Citations
- Precalculus. Authored by: OpenStax College. Provided by: OpenStax. Located at: http://cnx.org/contents/fd53eae1-fa23-47c7-bb1b-972349835c3c@5.175:1/Preface. License: CC BY: Attribution