Which conic section is formed when the plane cuts only one nappe of the cone and passes through the base?

A conic section is the plane curve formed by the intersection of a plane and a right-circular, two-napped cone. Such a cone is shown in Figure 1.

The cone is the surface formed by all the lines passing through a circle and a point. The point must lie on a line, called the "axis," which is perpendicular to the plane of the circle at the circle's center. The point is called the "vertex," and each line on the cone is called a "generatrix." The two parts of the cone lying on either side of the vertex are called "nappes." When the intersecting plane is perpendicular to the axis, the conic section is a circle (Figure 2).

Figure 1. Illustration by Hans & Cassidy. Courtesy of Gale Group.

Figure 3. Illustration by Hans & Cassidy. Courtesy of Gale Group.

When the intersecting plane is tilted and cuts completely across one of the nappes, the section is an oval called an ellipse (Figure 3).

When the intersecting plane is parallel to one of the generatrices, it cuts only one nappe. The section is an open curve called a parabola (Figure 4).

When the intersecting plane cuts both nappes, the section is a hyperbola, a curve with two parts, called "branches" (Figure 5).

All these sections are curved. If the intersecting plane passes through the vertex, however, the section will be a single point, a single line, of a pair of crossed lines. Such sections are of minor importance and are known as "degenerate" conic sections.

Figure 2. Illustration by Hans & Cassidy. Courtesy of Gale Group.

Figure 4. Illustration by Hans & Cassidy. Courtesy of Gale Group.

Figure 5. Illustration by Hans & Cassidy. Courtesy of Gale Group.

Figure 6. Illustration by Hans & Cassidy. Courtesy of Gale Group.

Figure 8. Illustration by Hans & Cassidy. Courtesy of Gale Group.

Since ancient times, mathematicians have known that conic sections can be defined in ways that have no obvious connection with conic sections. One set of ways is the following:

Ellipse: The set of points P such that PF1 + PF2 equals a constant and F1 and F2 are fixed points called the "foci" (Figure 6).

Parabola: The set of points P such that PD = PF, where F is a fixed point called the "focus" and D is the foot of the perpendicular from P to a fixed line called the "directrix" (Figure 7).

Hyperbola: The set of points P such that PF1 -PF2 equals a constant and F1 and F2 are fixed points called the "foci" (Figure 8).

If P, F, and D are shown as in Figure 7, then the set of points P satisfying the equation PF/PD = e where e is a constant, is a conic section. If 0 < e < 1, then the section is an ellipse. If e = 1, then the section is a parabola. If e > 1, then the section is a hyperbola. The constant e is called the "eccentricity" of the conic section.

Because the ratio PF/PD is not changed by a change in the scale used to measure PF and PD, all conic sections having the same eccentricity are geometrically similar.

Figure 7. Illustration by Hans & Cassidy. Courtesy of Gale Group.

Figure 9. Illustration by Hans & Cassidy. Courtesy of Gale Group.

Conic sections can also be defined analytically, that is, as points (x,y) which satisfy a suitable equation.

An interesting way to accomplish this is to start with a suitably placed cone in coordinate space. A cone with its vertex at the origin and with its axis coinciding with the z-axis has the equation x2 + y2 -kz2 = 0. The equation of a plane in space is ax + by + cz + d = 0. If one uses substitution to eliminate z from these equations, and combines like terms, the result is an equation of the form Ax2+ Bxy + Cy2 + Dx + Ey + F = 0 where at least one of the coefficients A, B, and C will be different from zero.

For example if the cone x2 + y2 -z2 = 0 is cut by the plane y + z - 2 = 0, the points common to both must satisfy the equation x2 + 4y - 4 = 0, which can be simplified by a translation of axes to x2 + 4y = 0. Because, in this example, the plane is parallel to one of the generatrices of the cone, the section is a parabola (Figure 9).

Figure 11. Illustration by Hans & Cassidy. Courtesy of Gale Group.

Figure 10. Illustration by Hans & Cassidy. Courtesy of Gale Group.

One can follow this procedure with other intersecting planes. The plane z - 5 = 0 produces the circle x2 + y2 - 25 = 0. The planes y + 2z - 2 = 0 and 2y + z - 2 = 0 produce the ellipse 12x2 + 9y2 - 16 = 0 and the hyperbola 3x2 - 9y2 + 4 = 0 respectively (after a simplifying translation of the axes). These planes, looking down the x-axis are shown in Figure 10.

As these examples illustrate, suitably placed conic sections have equations which can be put into the following forms:

The equations above are "suitably placed." When the equation is not in one of the forms above, it can be hard to tell exactly what kind of conic section the equation represents. There is a simple test, however, which can do this. With the equation written Ax2 + Bxy + Cy2 + Dx + Ey + F = 0, the discriminant B2 - 4AC will identify which conic section it is. If the discriminant is positive, the section is a hyperbola; if it is negative, the section is an ellipse; if it is zero, the section is a parabola. The discriminant will not distinguish between a proper conic section and a degenerate one such as x2 - y2 = 0; it will not distinguish between an equation that has real roots and one, such as x2 + y2 + 1 = 0, that does not.

Students who are familiar with the quadratic formula

will recognize the discriminant, and with good reason. It has to do with finding the points where the conic section crosses the line at infinity. If the discriminant is negative, there will be no solution, which is consistent with the fact that both circles and ellipses lie entirely within the finite part of the plane. Parabolas lead to a single root and are tangent to the line at infinity. Hyperbolas lead to two roots and cross it in two places.

Conic sections can also be described with polar coordinates. To do this most easily, one uses the focus-directrix definitions, placing the focus at the origin and the directrix at x = -k (in rectangular coordinates). Then the polar equation is r = Ke/(1 - e cos θ) where e is the eccentricity (Figure 11).

The eccentricity in this equation is numerically equal to the eccentricity given by another ratio: the ratio CF/CV, where CF represents the distance from the geometric center of the conic section to the focus and CV the distance from the center to the vertex. In the case of a circle, the center and the foci are one and the same point; so CF and the eccentricity are both zero. In the case of the ellipse, the vertices are end points of the major axis, hence are farther from the center than the foci. CV is therefore bigger than CF, and the eccentricity is less than 1. In the case of the hyperbola, the vertices lie on the transverse axis, between the foci, hence the eccentricity is greater than 1. In the case of the parabola, the "center" is infinitely far from both the focus and the vertex; so (for those who have a good imagination) the ratio CF/CV is 1.

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A conic section is the intersection of a plane and a double right circular cone .  By changing the angle and location of the intersection, we can produce different types of conics. There are four basic types: circles , ellipses , hyperbolas and parabolas .    None of the intersections will pass through the vertices of the cone.

If the right circular cone is cut by a plane perpendicular to the axis of the cone, the intersection is a circle.  If the plane intersects one of the pieces of the cone and its axis but is not perpendicular to the axis, the intersection will be an ellipse.  To generate a parabola, the intersecting plane must be parallel to one side of the cone and it should intersect one piece of the double cone. And finally, to generate a hyperbola the plane intersects both pieces of the cone. For this, the slope of the intersecting plane should be greater than that of the cone.

The general equation for any conic section is

A x 2 + B x y + C y 2 + D x + E y + F = 0 where A , B , C , D , E and F are constants.

As we change the values of some of the constants, the shape of the corresponding conic will also change.  It is important to know the differences in the equations to help quickly identify the type of conic that is represented by a given equation.
      If B 2 − 4 A C is less than zero, if a conic exists, it will be either a circle or an ellipse.
      If B 2 − 4 A C equals zero, if a conic exists, it will be a parabola.
      If B 2 − 4 A C is greater than zero, if a conic exists, it will be a hyperbola.

STANDARD FORMS OF EQUATIONS OF CONIC SECTIONS:

Solving Systems of Equations

You must be familiar with solving system of linear equation . Geometrically it gives the point(s) of intersection of two or more straight lines. In a similar way, the solutions of system of quadratic equations would give the points of intersection of two or more conics.

Algebraically a system of quadratic equations can be solved by elimination or substitution just as in the case of linear systems.

Example:

Solve the system of equations.

x 2 + 4 y 2 = 16 x 2 + y 2 = 9

The coefficient of x 2 is the same for both the equations. So, subtract the second equation from the first to eliminate the variable x . You get:

3 y 2 = 7

Solving for y :

3 y 2 3 = 7 3       y 2 = 7 3           y = ± 7 3

Use the value of y to evaluate x .

x 2 + 7 3 = 9 x 2 = 9 − 7 3             = 20 3     x = ± 20 3

Therefore, the solutions are ( + 20 3 , + 7 3 ) , ( + 20 3 , − 7 3 ) , ( − 20 3 , + 7 3 ) and ( − 20 3 , − 7 3 ) .

Now, let us look at it from a geometric point of view.

If you divide both sides of the first equation x 2 + 4 y 2 = 16 by 16 you get x 2 16 + y 2 4 = 1 . That is, it is an ellipse centered at origin with major axis 4 and minor axis 2 . The second equation is a circle centered at origin and has a radius 3 . The circle and the ellipse meet at four different points as shown.