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Hyperbolic Mirrors Hyperbolas have interesting reflective properties that make them useful for lenses and mirrors. For example, if a ray of light strikes a convex hyperbolic mirror on a line that would (theoretically) pass through its rear focus, it is reflected through the front focus. This property, and that of the parabola, were used to develop the Cassegrain telescope in 1672. The focus of the parabolic mirror and the rear focus of the hyperbolic mirror are the same point. The rays are collected by the parabolic mirror, reflected toward the (common) focus, and thus are reflected by the hyperbolic mirror through the opening to its front focus, where the eyepiece is located. If the equation of the hyperbola is y29-x216=1 and the focal length (distance from the vertex to the focus) of the parabola is 6, find the equation of the parabola.

Short Answer

Expert verified

The focal length, a, is 6and the vertex of the parabola is in (0,-11), or x=0and y=-11, it follows that the parabola equation equals

x2=24(y+11)

Step by step solution

01

Step 1. Analyzing the equation of the hyperbola

The hyperbola is given with the following equation

y29-x216=1

Since the variable xis being subtracted from the variable yit follow tha the transverse axis of the hyperbola is parallel to the y-axis

A hyperbola with its transverse axis parallel to the y-axis and its center in the point (h,k)has the following equation form:

(y-k)2a2-(x-h)2b2=1

By comparing, the center of the given hyperbola is in (0,0)

a2=9⇒a=3

and

b2=16⇒b=4

02

Step 2. Analyze the hyperbola

A hyperbola its transverse axis parallel to the y-axis and its origin. (0,0)

(0,-a)and (0,a)

Since you know a=3it follow that the vertices of the hyperbola are located in

(0,-3)and (0,3)

Again,

A hyperbola with its transverse axis parallel to the y-axis and its center in the origin, (0,0)

(0,c)and (0,-c)

Now,

c2=32+42=9+16=25⇒c=5

its foci are then

(0,5)and(0,-5)

03

Step 3. Make a drawing

You can make a drawing of the given hyperbola:

04

Step 4. Finding the parabola equation

From the text of the exercise you know that the rear focus of the hyperbola is the same point as the focus of the parabola.

Since the focal length, the distance from the vertex of the parabola to the focus is 6, it follow that the directrix of the parabola is 6unit away from the vertex of the parabola

Then the vertex of the parabola is located in the point (0,-11)and the directrix is the line y=-17

A standard equation of a parabola with its of symmetry parallel to the y-axis is

(x-h)2=4a(y-k)

Since the focal length,ais 6and the vertex of the parabola is in (0,-11)

x2=24(y+11)

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Most popular questions from this chapter

Solar Heat - A mirror is shaped like a paraboloid of

revolution and is used to concentrate the rays of the sun at

its focus, creating a heat source. See the figure. If the mirror

is 20 feet across at its opening and is 6 feet deep, where will

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(a) Find parametric equations that model the position of the ball as a function of time.

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(c) Determine the horizontal distance that the ball travels.

(d) When is the ball at its maximum height? Determine the maximum height of the ball.

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intersection when the Impala is 4 miles from the intersection. See the figure.

(a) Find parametric equations that model the motion of the Camry and Impala.

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(e) Simulate the motion of the cars by simultaneously graphing the equations found in part (a).

Find the vertex, focus, and directrix of each parabola. Graph the equation by hand. Verify your graph using a graphing utility.

y2=8x

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