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EXAMPLE 4Graphing a Function of Two Variables

Graph each function:

  1. (a) z=f(x,y)=1xy
  2. (b) z=f(x,y)=x2+4y2
  3. (c) z=f(x,y)=x2+y2

Planes in space are discussed in Section 10.6, pp. 737-740.

Solution (a) The graph of the equation z=1xy, or x+y+z=1, is a plane. The intercepts are the points (1,0,0), (0,1,0), and (0,0,1). See Figure 6.

(b) The graph of the equation z=x2+4y2 is an elliptic paraboloid whose vertex is at the origin. See Figure 7.

Quadric surfaces are discussed in Section 10.7, pp. 744-751.

(c) The equation z=f(x,y)=x2+y2 is equivalent to z2=x2+y2, where z0. The graph of the equation is part of a circular cone whose vertex is at the origin. Since z0, the graph of f is the upper nappe of the cone. See Figure 8.

Figure 6 z=f(x,y)=1xy
Figure 7 z=f(x,y)=x2+4y2
Figure 8 z=f(x,y)=x2+y2