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Conic Sections: Ellipses
Example 1 Write an Equation for a Graph
Write an equation for the ellipse.
To write the equation for an ellipse, we need to find the values of a and b for the ellipse. We know that the length of the major axis of any ellipse is 2 a units. In this ellipse, the length of the major axis is the distance between the points at (4, 0) and (–4, 0). This distance is 8 units. 2 a = 8 Length of major axis = 8. a = 4 Divide each side by 2.
The foci are located at (2 3 , 0) and (– 2 3 , 0),
so c = 2 3. We can use the relationship between a , b , and c to determine the value of b. c^2 = a^2 – b^2 Equation relating a , b , and c
12 = 16 – b^2 c = 2 3 and a = 4 b^2 = 4 Solve for b^2.
Since the major axis is horizontal, substitute 16 for a^2 and 4 for b^2 in the form
x^2 a^2
y^2 b^2
= 1. An equation
of the ellipse is
x^2 16
y^2 4
Example 2 Write an Equation Given the Lengths of the Axes
ARCHITECTURE In an ellipse, sound or light coming from one focus is reflected to the other focus. A retaining wall around a cul-de-sac is in the shape of an ellipse. A person can hear another person whisper from across the cul-de-sac if the two people are standing at the foci. The retaining wall has an elliptical cross section that is 14 feet 6 inches by 45 feet.
a. Write an equation to model this ellipse. Assume that the center is at the origin and the major axis is horizontal.
The length of the major axis is 45 feet. 2 a = 45 feet Length of major axis = 45 ft. a = 22.5 Divide each side by 2.
The length of the minor axis is 14
6 12 or^
174
2 b =
174 12 Length of minor axis =^
174
b =
29 4 Divide each side by 2.
Substitute a =
2 and^ b^ =
4 into the form
2 2
x y
a b
. An equation of the ellipse is
2 2
x y
b. How far apart are the points at which two people should stand to hear each other whisper? People should stand at the two foci of the ellipse. The distance between the foci is 2 c units. c^2 = a^2 - b^2 Equation relating a , b , and c
c = a^2^ b^2 Take the square root of each side.
2 c = 2 a^2^ b^2 Multiply each side by 2.
2 c = 2
2 2
Substitute a =
71 3 and^ b^ =
27
2 c ≈ 42.
The points where two people should stand to hear each other whisper are about 42.60 feet apart.
Example 4 Graph an Equation not in Standard Form
Find the coordinates of the center and foci and the lengths of the major and minor axes of the ellipse with equation 4 y^2 + 5 x^2 – 24 y + 20 x – 124 = 0. Then graph the ellipse.
Complete the square for each variable to write this equation in standard form. 4 y^2 + 5 x^2 – 24 y + 20 x – 124 = 0 Original equation 4( y^2 – 6 y + •) + 5( x^2 + 4 x + •) = 124 + 4(•) + 5(•) Complete the squares.
4( y^2 – 6 y + 9) + 5( x^2 + 4 x + 4) = 124 + 4(9) + 5(4)
2 6 2
= 9,
2 4 2
= 4
4( y – 3)^2 + 5( x + 2)^2 = 180 Write the trinomials as perfect squares.
( y – 3 )^2
45
( x 2 )^2
36
= 1 Divide each side by 180.
The center of this ellipse is at (–2, 3) and the foci are at (–2, 6) and (–2, 0). The length of the major
axis is 6 5 units, and the length of the minor axis is 12.
