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0 _1 1 _1
0 y 1 FIGURE 6 Vertical traces are parabolas;
horizontal traces are hyperbolas.
All traces are labeled with the
value of k. x x 0
2 1 Traces in x=k are z=¥k @ Traces in y=k are z=_≈+k @ Traces in z=k are ¥≈=k z z z 1 0
y 0
FIGURE 7 1 Traces moved to their
correct planes x _1 x x _1 _1 0 1 Traces in y=k Traces in x=k In Module 13.6A you can investigate
how traces determine the shape of a
surface. y y Traces in z=k In Figure 8 we ﬁt together the traces from Figure 7 to form the surface z y 2 x 2,
a hyperbolic paraboloid. Notice that the shape of the surface near the origin resembles
that of a saddle. This surface will be investigated further in Section 15.7 when we discuss saddle points.
z 0
y x FIGURE 8 The surface z=¥≈ is a
hyperbolic paraboloid.
EXAMPLE 6 Sketch the surface x2
4 y2 z2
4 SOLUTION The trace in any horizontal plane z x2
4 y2 1 1.
k is the ellipse
k2
4 z k 5E13(pp 868877) 872 ❙❙❙❙ 1/18/06 11:35 AM Page 872 CHAPTER 13 VECTORS AND THE GEOMETRY OF SPACE z but the traces in the x z and y zplanes are the hyperbolas
x2
4 1 y 0 y2 and z2
4 1 x 0 This surface is called a hyperboloid of one sheet and is sketched in Figure 9. (0, 1, 0) (2, 0, 0) z2
4 y x The idea of using traces to draw a surface is employed in threedimensional graphing
software for computers. In most such software, traces in the vertical planes x k and
y k are drawn for equally spaced values of k, and parts of the graph are eliminated using
hidden line removal. Table 1 shows computerdrawn graphs of the six basic types of
quadric surfaces in standard form. All surfaces are symmetric with respect to the zaxis. If
a quadric surface is symmetric about a different axis, its equation changes accordingly. FIGURE 9 TABLE 1 Graphs of quadric surfaces Surface Equation
x2
a2 Ellipsoid
z y2
b2 z2
c2 Surface
1 z x z
c Elliptic Paraboloid
z x2
a2 y2
b2 x2
a2 y2
b2 Horizontal traces are ellipses. c, the ellipsoid is y x z2
c2 Cone All traces are ellipses.
If a b
a sphere. Equation y Vertical traces in the p...
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 Fall '09
 hamrick

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