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9.6 Degenerate Conics

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    1037
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    The general equation of a conic is \(A x^{2}+B x y+C y^{2}+D x+E y+F=0\). This form is so general that it encompasses all regular lines, singular points and degenerate hyperbolas that

    look like an \(\mathrm{X}\). This is because there are a few special cases of how a plane can intersect a two sided cone. How are these degenerate shapeGraphing Degenerate Conicss formed?

    Graphing Degenerate Conics

    A degenerate conic is a conic that does not have the usual properties of a conic. Degenerate conic equations simply cannot be written in graphing form. There are three types of degenerate conics:

    1. A singular point, which is of the form: \(\frac{(x-h)^{2}}{a}+\frac{(y-k)^{2}}{b}=0\). You can think of a singular point as a circle or an ellipse with an infinitely small radius.

    2. A line, which has coefficients \(A=B=C=0\) in the general equation of a conic. The remaining portion of the equation is \(D x+E y+F=0,\) which is a line.

    3. A degenerate hyperbola, which is of the form: \(\frac{(x-h)^{2}}{a}-\frac{(y-k)^{2}}{b}=0\). The result is two intersecting lines that make an "X" shape. The slopes of the intersecting lines forming the \(\mathrm{X}\) are \(\pm \frac{b}{a}\). This is because \(b\) goes with the \(y\) portion of the equation and is the rise, while \(a\) goes with the \(x\) portion of the equation and is the run.

    Examples

    Example 1

    Earlier, you were asked how degenerate conics are formed. When you intersect a plane with a two sided cone where the two cones touch, the intersection is a single point. When you intersect a plane with a two sided cone so that the plane touches the edge of one cone, passes through the central point and continues touching the edge of the other conic, this produces a line. When you intersect a plane with a two sided cone so that the plane passes vertically through the central point of the two cones, it produces a degenerate hyperbola.

    Example 2

    Transform the conic equation into standard form and sketch.

    \(0 x^{2}+0 x y+0 y^{2}+2 x+4 y-6=0\)

    This is the line \(y=-\frac{1}{2} x+\frac{3}{2}\).

    Example 3

    Transform the conic equation into standard form and sketch.

    \(3 x^{2}-12 x+4 y^{2}-8 y+16=0\)

    \(3 x^{2}-12 x+4 y^{2}-8 y+16=0\)

    \(\begin{aligned} 3\left(x^{2}-4 x\right)+4\left(y^{2}-2 y\right) &=-16 \\ 3\left(x^{2}-4 x+4\right)+4\left(y^{2}-2 y+1\right) &=-16+12+4 \\ 3(x-2)^{2}+4(y-1)^{2} &=0 \\ \frac{(x-2)^{2}}{4}+\frac{(y-1)^{2}}{3} &=0 \end{aligned}\)

    The point (2,1) is the result of this degenerate conic.

    Example 4

    Transform the conic equation into standard form and sketch.

    \(16 x^{2}-96 x-9 y^{2}+18 y+135=0\)

    \(16 x^{2}-96 x-9 y^{2}+18 y+135=0\)

    \(\begin{aligned} 16\left(x^{2}-6 x\right)-9\left(y^{2}-2 y\right) &=-135 \\ 16\left(x^{2}-6 x+9\right)-9\left(y^{2}-2 y+1\right) &=-135+144-9 \\ 16(x-3)^{2}-9(y-1)^{2} &=0 \\ \frac{(x-3)^{2}}{9}-\frac{(y-1)^{2}}{16} &=0 \end{aligned}\)

    This is a degenerate hyperbola.

    Example 5

    1. Create a conic that describes just the point (4,7) ).

    \((x-4)^{2}+(y-7)^{2}=0\)

    Review

    1. What are the three degenerate conics?

    Change each equation into graphing form and state what type of conic or degenerate conic it is.

    2. \(x^{2}-6 x-9 y^{2}-54 y-72=0\)

    3. \(4 x^{2}+16 x-9 y^{2}+18 y-29=0\)

    4. \(9 x^{2}+36 x+4 y^{2}-24 y+72=0\)

    5. \(9 x^{2}+36 x+4 y^{2}-24 y+36=0\)

    6. \(0 x^{2}+5 x+0 y^{2}-2 y+1=0\)

    7. \(x^{2}+4 x-y+8=0\)

    8. \(x^{2}-2 x+y^{2}-6 y+6=0\)

    9. \(x^{2}-2 x-4 y^{2}+24 y-35=0\)

    10. \(x^{2}-2 x+4 y^{2}-24 y+33=0\)

    Sketch each conic or degenerate conic.

    11. \(\frac{(x+2)^{2}}{4}+\frac{(y-3)^{2}}{9}=0\)

    12. \(\frac{(x-3)^{2}}{9}+\frac{(y+3)^{2}}{16}=1\)

    13. \(\frac{(x+2)^{2}}{9}-\frac{(y-1)^{2}}{4}=1\)

    14. \(\frac{(x-3)^{2}}{9}-\frac{(y+3)^{2}}{4}=0\)

    15. \(3 x+4 y=12\)


    9.6 Degenerate Conics is shared under a CK-12 license and was authored, remixed, and/or curated by CK-12 Foundation via source content that was edited to conform to the style and standards of the LibreTexts platform; a detailed edit history is available upon request.

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