James Stewart-Calculus_ Early Transcendentals-Cengage Learning (2015)

Section 10.5 Conic Sections 677
y
(0, a)
(0, c)
4 The ellipse
x 2
a 2 1 y 2
b 2 − 1 a > b . 0
has foci s6c, 0d, where c 2 − a 2 2 b 2 , and vertices s6a, 0d.
(_b, 0)
(b, 0)
0 x
(0, _c)
If the foci of an ellipse are located on the y-axis at s0, 6cd, then we can find its equation
by interchanging x and y in (4). (See Figure 9.)
(0, _a)
FIGURE 9
x 2
b 1 y 2
2 a − 1, a > b
2
5 The ellipse
x 2
b 2 1 y 2
a 2 − 1 a > b . 0
has foci s0, 6cd, where c 2 − a 2 2 b 2 , and vertices s0, 6ad.
y
(0, 3)
Example 2 Sketch the graph of 9x 2 1 16y 2 − 144 and locate the foci.
SOLUtion Divide both sides of the equation by 144:
(_4, 0)
x 2
16 1 y 2
9 − 1
{_œ„7, 0}
(4, 0)
0
{œ„7, 0} x
(0, _3)
The equation is now in the standard form for an ellipse, so we have a 2 − 16, b 2 − 9,
a − 4, and b − 3. The x-intercepts are 64 and the y-intercepts are 63. Also,
c 2 − a 2 2 b 2 − 7, so c − s7 and the foci are s6s7 , 0d. The graph is sketched in
Figure 10.
n
FIGURE 10
9x 2 1 16y 2 − 144
Example 3 Find an equation of the ellipse with foci s0, 62d and vertices s0, 63d.
SOLUtion Using the notation of (5), we have c − 2 and a − 3. Then we obtain
b 2 − a 2 2 c 2 − 9 2 4 − 5, so an equation of the ellipse is
x 2
5 1 y 2
9 − 1
Another way of writing the equation is 9x 2 1 5y 2 − 45.
n
F¡(_c, 0) 0 F(c, 0) x
FIGURE 11
P is on the hyperbola when
| PF1 | 2 | PF2 | − 62a.
y
P(x, y)
Like parabolas, ellipses have an interesting reflection property that has practical conse
quences. If a source of light or sound is placed at one focus of a surface with elliptical
cross-sections, then all the light or sound is reflected off the surface to the other focus
(see Exercise 65). This principle is used in lithotripsy, a treatment for kidney stones.
A reflector with elliptical cross-section is placed in such a way that the kidney stone is
at one focus. High-intensity sound waves generated at the other focus are reflected to
the stone and destroy it without damaging surrounding tissue. The patient is spared the
trauma of surgery and recovers within a few days.
Hyperbolas
A hyperbola is the set of all points in a plane the difference of whose distances from two
fixed points F 1 and F 2 (the foci) is a constant. This definition is illustrated in Figure 11.
Hyperbolas occur frequently as graphs of equations in chemistry, physics, biology,
and economics (Boyle’s Law, Ohm’s Law, supply and demand curves). A particularly
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