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Chapter Eleven Partial Derivatives Section 11.1 Functions of Several Variables Goals Study functions of two or more variables from four points of view Discuss visual representations Describe functions of three or more variables Four Points of View We can study functions of two or more variables from four viewpoints: Verbally Numerically Algebraically Visually Example The wind-chill index is often used to describe the apparent severity of the cold. The index W depends on the actual temperature T and the wind speed v. A table of values of W(T, v). f(–5, 50) = –15. Domain and Range Recall that for a function f(x, y) given by an algebraic formula, the domain consists of all pairs (x, y) for which the expression for f(x, y) is a well-defined real number. As an example we find the domain and range of g x , y 9x y : 2 2 Solution The domain of g is D x , y |9 x y 0 x , y | x y 9 2 2 2 2 This is the disk with center (0, 0) and radius 3. The range of g is z| z 9 x y , x , y D 2 Since z is a positive square root, z ≥ 0. Also 9x y 9 2 2 2 9x y 3 2 2 So the range is z |0 z 3 0 ,3 . Visual Representations One way to visualize a function of two variables is through its graph. For example, we sketch the graph of g x , y 9x y : 2 2 Solution Squaring both sides of this equation gives x2 + y2 + z2 = 9, which is an equation of the sphere with center the origin and radius 3. Since z ≥ 0, the graph of g is just the top half of this sphere. Level Curves Another way to see functions is a contour map on which points of constant elevation are joined to form level curves: The level curves f(x, y) = k are traces of the graph of f in the plane z = k projected down to the xy-plane: A common example of level curves occurs in topographical maps of mountainous regions, as shown on the next slide. The level curves are curves of constant elevation above sea level. If we walk along one of these contour lines we neither ascend nor descend. Example The figure shows a contour map for a function. Use it to estimate f(1, 3) and f(4, 5). The point (1, 3) lies part way between the level curves with z-values 70 and 80. So we estimate that f (1, 3) ≈ 73. Similarly, we estimate that f (4, 5) ≈ 56. Example Sketch the level curves of the function g x , y 9 x y for k 0 ,1,2 ,3 2 2 Solution The level curves are 9 x y k or x y 9 k 2 2 2 2 2 This is a family of concentric circles with center (0, 0) and 2 radius 9k . The cases k = 0, 1, 2, 3 are shown: Example Sketch some level curves of the function h(x, y) = 4x2 + y2. Solution The level curves are 4 x y k or 2 2 x 2 k/4 y 2 k 1 which, for k > 0, describes a family of ellipses with semiaxes k / 2 and k . Shown below are these level curves lifted up to the graph of h: Three or More Variables A function of three variables, f, is a rule that assigns to each ordered triple (x, y, z) in a domain D in space a unique real number denoted by f(x, y, z). For instance, the temperature T at a point on the surface of the Earth depends on the longitude x and latitude y of the point and on the time t, so T = f(x, y, t). Example Find the domain of f if f(x, y, z) = ln (z – y) + xy sin z. Solution The expression for f(x, y, z) is defined as long as z – y > 0, so the domain of f is the half-space Level Surfaces To gain insight into a function f of three variables we can examine its level surfaces, which are the surfaces with equations f(x, y, z) = k, where k is a constant. If the point (x, y, z) moves along a level surface, the value of f(x, y, z) remains fixed. Example Find the level surfaces of the function f(x, y, z) = x2 + y2 + z2 Solution The level surfaces are x2 + y2 + z2 = k, where k ≥ 0. These form a family of concentric spheres with radius k , as the next slide shows : More Variables Functions of any number of variables can be considered. A function of n variables is a rule that assigns a number z = f(x1, x2,…, xn) to an n-tuple (x1, x2,…, xn) of real numbers. Sometimes we use vector notation in order to write functions more compactly: If x x , x , , x , we often write f(x) in place of f(x1, x2,…, xn). So there are three ways of viewing a function of n variables: As a function of… 1 2 n n real variables x1, x2,…, xn ; a single point variable (x1, x2,…, xn); a single vector variable x x1 , x2 , , xn Review Four ways of viewing functions of two variables Visual representations Graphs Level curves Functions of three or more variables Level surfaces