# PROBLEM SET. Practice Problems for Exam #2. Math 2350, Fall Nov. 7, 2004 Corrected Nov. 10 ANSWERS

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1 PROBLEM SET Practice Problems for Exam #2 Math 2350, Fall 2004 Nov. 7, 2004 Corrected Nov. 10 ANSWERS

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3 Problem 1. Consider the function f(x, y) = xy 2 sin(x 2 y). Find the partial derivatives f x, f y, f xx, f xy, f yx and f yy. For the first order derivatives, we have and f x (x, y) = y 2 sin(x 2 y) + xy 2 cos(x 2 y)2xy = y 2 sin(x 2 y) + 2x 2 y 3 cos(x 2 y), f y (x, y) = 2xy sin(x 2 y) + xy 2 cos(x 2 y)x 2 = 2xy sin(x 2 y) + x 3 y 2 cos(x 2 y).. For the second order derivatives, we have f xx (x, y) = x [y2 sin(x 2 y) + 2x 2 y 3 cos(x 2 y)] = y 2 cos(x 2 y)2xy + 4xy 2 cos(x 2 y) 2x 2 y 3 sin(x 2 y)2xy = 2xy 3 cos(x 2 y) + 4xy 3 cos(x 2 y) 4x 3 y 4 sin(x 2 y) f xy (x, y) = y [y2 sin(x 2 y) + 2x 2 y 3 cos(x 2 y)] = 2y sin(x 2 y) + y 2 cos(x 2 y)x 2 + 6x 2 y 2 cos(x 2 y) 2x 2 y 3 sin(x 2 y)x 2 = 2y sin(x 2 y) + 7x 2 y 2 cos(x 2 y) 2x 4 y 3 sin(x 2 y) f yx (x, y) = x [2xy sin(x2 y) + x 3 y 2 cos(x 2 y)] = 2y sin(x 2 y) + 2xy cos(x 2 y)2xy + 3x 2 y 2 cos(x 2 y) x 3 y 2 sin(x 2 y)2xy = 2y sin(x 2 y) + 7x 2 y 2 cos(x 2 y) 2x 4 y 3 sin(x 2 y) f yy (x, y) = y [2xy sin(x2 y) + x 3 y 2 cos(x 2 y)] = 2x sin(x 2 y) + 2xy cos(x 2 y)x 2 + 2x 3 y cos(x 2 y) x 3 y 2 sin(x 2 y)x 2 = 2x sin(x 2 y) + 4x 3 y cos(x 2 y) x 4 y 2 sin(x 2 y). Problem 2. Use the chain rule for partial derivatives to find r/ x and r/ y where r = uv u 2 v 2, u = x 2 y and v = 2x y. Express your answer in terms of x and y. 1

4 We have By the chain rule, Similarly, we have r u = v 2u r v = u 2v u x = 2xy u y = x2 v x = 2 v y = 1 r x = r u u x + r v v x = (v 2u)2xy + (u 2v)2 = (2x y 2x 2 y)2xy + (x 2 y 4x + 2y)2 = 4x 2 y 2xy 3 4x 3 y 2 + 2x 2 y 8x + 4y = 6x 2 y 2xy 3 4x 3 y 2 8x + 4y r y = r u u y + r v v y = (v 2u)x 2 + (u 2v)( 1) = (2x y 2x 2 y)x 2 (x 2 y 4x + 2y) = 2x 3 x 2 y 2x 4 y x 2 y + 4x 2y = 2x 3 2x 2 y 2x 4 y + 4x 2y. Problem 3. Suppose that p = f(u, v, w), u = g(x, y), v = h(y, z) and w = k(x, y, z). Write the chain rule formals for the partial derivatives of p with respect to x, y and z. p x = p u u x + p w w x p y = p u u y + p v v y + p w w y p z = p v v z + p w w z. 2

5 Problem 4. A box is measured to have a length of 15 inches, a width of 10 inches and a depth of 5 inches. From these measurements, we would calculate the volume to be 750 cubic inches. If there is a possible error of up to 0.25 inch in the measurements, use differentials to estimate the maximum possible error in the calculated volume. Let x, y and z denote the length, width and depth of the box. Then the volume is given by V = xyz. The total differential of the volume is dv = V V dx + x y dy + V z dz = yz dx + xz dy + xy dz. To approximate the error in the volume, we plug in x = 15, y = 10, z = 5 and dx = dy = dz = Thus, cubic inches. Error in Volume dv = (10)(5)(0.25) + (15)(5)(0.25) + (15)(10)(0.25) = Problem 5. A rectangular block of ice is melting in the sun. At a certain instant the block has a height of 5 inches, a length of 10 inches, and a width of 12 inches, and each of the dimensions is decreasing at a rate of 2 inches per hour. How fast is the volume of the block changing? Let x be the height of the box, y the length and z the width so the volume of the box is V = xyz. By the chain rule At the given instant, we have dv dt = V dx x dt + V dy y dt + V dz z dt = yz dx dy dz + xz + xy dt dt dt x = 5, y = 10, z = 12, dx dt = dy dt = dz dt = 2 3

6 (minus since the dimensions are decreasing). Thus, at this instant, dv dt = (10)(12)( 2) + (5)(12)( 2) + (5)(10)( 2) = 460, so the volume is decreasing at a rate of 460 cubic inches per hour. Problem 6. Find the directional derivative of the function f(x, y) = x 2 + 2xy at the point (1, 2) in the direction of the vector v = 3i + 4j. The gradient of f is given by f(x, y) = f x (x, y)i + f y (x, y)j = (2x + 2y)i + 2xj and so The length of the vector v is f(1, 2) = 6i + 2j. v = = 5, so a unit vector in the direction of v is u = 1 5 v = 3 5 i j. Thus, the directional derivative we want is D u f(1, 2) = f(1, 2) u ( 3 = (6i + 2j) 5 i + 4 ) 5 j = 26/5. Problem 7. Suppose that the temperature at a point (x, y) in the plane is given by f(x, y) = x 2 + xy + y 2. A bug is walking around in the plane with a speed of 5 units per minute. A. At a certain instant the bug is at the point (2, 1) and has velocity vector v = (3, 4). What rate of change of temperature is the bug experiencing at this instant? 4

7 If the bug s position at time t is (x(t), y(t)), the bug s temperature at time t is T (t) = f(x(t), y(t)). The rate of change of the bug s temperature is dt (t) = f(x(t), y(t)) v(t), dt where v(t) is the bug s velocity vector. We have f(x, y) = (2x + y, x + 2y). At our instant, call it t = t 0, we have (x(t 0 ), y(t 0 )) = (2, 1) and v(t 0 ) = (3, 4). Thus, we have dt dt (t 0) = f(x(t 0 ), y(t 0 )) v(t 0 ) = f(2, 1) (3, 4) = (5, 4) (3, 4) = = 1. B. Suppose the bug is at (2, 1). If the bug wants to cool off, what direction should he travel (at speed 5) to get fastest decrease in temperature? What is this rate of change of temperature? The direction of fastest decrease is the direction of f(2, 1). The vector of length 5 in this direction is 5 v = f(2, 1), f(2, 1) so this should be the bug s velocity vector. The rate of change of temperature will be dt dt = f(2, 1) v [ 5 = f(2, 1) f(2, 1) f(2, 1) 2 = 5 f(2, 1) = 5 f(2, 1) = ] f(2, 1) 5

8 Problem 8. Find the equation of the tangent plane to the cone z 2 = x 2 + y 2 at the point P = (4, 3, 5). Find parametric equations for the normal line to the surface at this point. The cone is the level surface The gradient of this function is g(x, y, z) = x 2 + y 2 z 2. g(x, y, z) = (2x, 2y, 2z). Since g is orthogonal to the level surfaces of g, the vector N = g(4, 3, 5) = (8, 6, 10) is perpendicular to the surface (i.e., perpendicular to the tangent plane of the surface) at P. Hence the tangent plane is the plane that passes through the point with position vector x 0 = (4, 3, 5) and is perpendicular to N. The vector form of the equation is where x = (x, y, z). Thus, we have 0 = N (x x 0 ) N (x x 0 ) = 0 = N [(x, y, z) (4, 3, 5)] = N (x 4, y + 3, z 5) = (8, 6, 10) (x 4, y + 3, z 5) = 8(x 4) 6(y + 3) 10(z 5). Thus, the equation of the tangent plane is 8(x 4) 6(y + 3) 10(z 5) = 0, which can be simplified to standard form as 4x 3y 5z = 0. The normal line passes through P and is parallel to g(p ) = N, so the vector form of the parametrization is x = x 0 + tn = (4, 3, 5) + t(8, 6, 10) = (4 + 8t, 3 6t, 5 10t), so the parametric equations are x = 4 + 8t y = 3 6t z = 5 10t 6

9 Problem 9. Locate an classify the critical points of the function f(x, y) = x 4 + 4xy + y 4. We can calculate the partial derivatives as follows f x (x, y) = 4x 3 + 4y f y (x, y) = 4x + 4y 3 f xx (x, y) = 12x 2 f xy (x, y) = 4 f yy (x, y) = 12y 2. To find the critical points we want to solve the equations f x (x, y) = 0 and f y (x, y) = 0. We can divide both equations by 4, so we want to solve the system of equations (9.1) (9.2) From (9.1) we have x 3 + y = 0 x + y 3 = 0 (9.3) y = x 3. Plugging this into (9.2) gives x + ( x 3 ) 3 = 0 or x x 9 = 0. This factors as x(1 x 8 ) = 0. The solutions of this equation are x = 0 and x = ± 8 1 = ±1. Plugging these values of x into (9.3) we find three critical points (0, 0), (1, 1) and (-1,1). Consider the critical point (0, 0). At this point, the value of the discriminant D = f xx f yy f 2 xy is 16 < 0, so (0, 0) is a saddle point. At (1, 1) the value of the discriminant is D = > 0, so we have a relative extrema. We have f xx > 0 at this point, so the graph is concave up in the x-direction. Thus (1, 1) is a relative min. Similarly, we have D = > 0 and f xx > 0 at ( 1, 1), so ( 1, 1) is a relative min. Problem 10. Find the absolute max and min of the function f(x, y) = xy on the disk x 2 + y 2 2. You can parametrize the boundary, or use Lagrange multipliers. The region is the disk of radius 2 centered at the origin. First we find the critical points in the interior of the region. The partials are f x (x, y) = y f y (x, y) = x. 7

10 Setting these equal to zero gives the system of equations y = 0 x = 0. Thus, we get one critical point (0, 0) of f which is in the interior of the region. We have f(0, 0) = 0. Now we have to find the max and min of f on the boundary circle x 2 +y 2 = 2. You could do this by parameterizing the circle by x = 2 cos(θ) and y = 2 sin(θ). I ll do it by Lagrange multipliers. Thus we want to maximize and minimize f(x, y) = xy subject to the constraint g(x, y) = x 2 + y 2 = 2. The Lagrange multiplier equations are f = λ g and g = 2 in vector form. In order words f x = λg x f y = λg y g = 2. Plugging in the formulas we get this system (10.1) (10.2) (10.3) y = 2λx x = 2λy x 2 + y 2 = 2. Plugging (10.1) into (10.2) gives x = 4λ 2 x or (10.4) (4λ 2 1)x = 0. First, consider the case where 4λ Then (10.4) implies that x = 0. But then (10.1) implies that y = 0. The point (0, 0) does not satisfy the constraint equation (10.3), so there is no solution of the system with 4λ Thus, we must have 4λ 2 1 = 0, i.e., λ = ±1/2. Consider first the case λ = 1/2. Then equation (10.1) and equation (10.2) become y = x. Substituting this into the constraint (10.3) gives 2x 2 = 2, so x = ±1. Since y = x, we get two critical points (1, 1) and ( 1, 1). The value of the function at these points is f = 1. In the case λ = 1/2, (10.1) gives y = x. Plugging this into the constraint equation again gives 2x 2 = 2, so x = ±1. Thus, we get two critical points (1, 1) and ( 1, 1). The value of the function at these points is f = 1. At the points that are candidates for the absolute max and min of f, we have values 1, 0, 1. Thus, the absolute min is of f on the disc is 1, which occurs at the points (1, 1) and ( 1, 1). The absolute max of f on the disk is 1, which occurs at the points (1, 1) and ( 1, 1). 8

11 Problem 11. Find the absolute max and min of the function f(x, y, z) = xyz 2 on the spherical surface x 2 + y 2 + z 2 = 128. (The absolute max and min exist because the surface is closed and bounded.) The problem is to extermize the function f(x, y, z) = xyz 2 subject to the constraint g(x, y, z) = x 2 + y 2 + z 2 = 128. The Lagrange multiplier equations are f = λ g and g = 128. Plugging in the formulas for the partial derivatives this is equivalent to (11.1) (11.2) (11.3) (11.4) yz 2 = 2λx xz 2 = 2λy 2xyz = 2λz = xyz = λz. x 2 + y 2 + z 2 = 128 Now multiply (11.1) by x, (11.2) by y, and (11.3) by z to get (11.5) (11.6) (11.7) xyz 2 = 2λx 2 xyz 2 = 2λy 2 xyz 2 = λz 2. Thus, we have (11.8) 2λx 2 = 2λy 2 = λz 2. Consider first the case λ = 0. Then, from (11.1) through (11.3) we must have xz 2 = 0, yz 2 = 0 and xyz = 0. These equations are satisfied if z = 0. Thus, all the points on the circle x 2 + y 2 = 128 in the plane z = 0 are critical points. The value of the function f at these points is zero. If z 0, we must have x = y = 0. From the constraint, this implies that z 2 = 128, so z = ±8 2. The function is zero at these points. If λ 0, the we can divide (11.8) by λ to get (11.9) 2x 2 = 2y 2 = z 2. Multiplying the constraint equation by 2 gives 2x 2 + 2y 2 + 2z 2 = 256 Substituting z 2 for 2z 2 and 2y 2 gives 4z 2 = 256, so z = ±8. We then have 2x 2 = z 2 = 64, so x = ± 32 = ±4 2. Similarly, y = ±4 2. Thus, we get 8 critical points (±4 2, ±4 2, ±8). The values of the function at these points are ±256. Thus, the absolute min is 256 and the absolute max is 256. I ll leave it to the reader to list the points at which these occur. 9

12 Problem 12. Consider the curve C formed by the intersection of the cylinder x 2 + y 2 = 1 and the plane x + y + z = 1. Find the points on the curve that are closest and farthest from the origin. (The curve is closed and bounded.) I ll use the usual trick of extermizing the square of the distance from the origin. Thus, our problem is to extermize f(x, y, z) = x 2 + y 2 + z 2 subject to the constraints g 1 (x, y, z) = x 2 + y 2 = 1 and g 2 (x, y, z) = x + y + z = 1. The Lagrange multiplier equations are f = λ 1 g 1 + λ 2 g 2 g 1 = 1 g 2 = 1. Writing these out gives the system of equations These can be rewritten as 2x = 2λ 1 x + λ 2 2y = 2λy + λ 2 2z = λ 2 x 2 + y 2 = 1 x + y + z = 1 (12.1) (12.2) (12.3) (12.4) (12.5) 2(1 λ 1 )x = λ 2 2(1 λ 1 )y = λ 2 2z = λ 2 x 2 + y 2 = 1 x + y + z = 1 Consider first the case λ 1 = 1. Then (12.1) and (12.2) imply that λ 2 = 0. Plugging this into (12.3) gives z = 0. Plugging z = 0 into (12.4) and (12.5) gives the equations x 2 + y 2 = 1 and x + y = 1. Substitute y = 1 x into x 2 + y 2 = 1. Then we have x 2 + (1 x) 2 = x x + x 2 = 1, which reduces to x(x 1) = 0, so we have solutions x = 0 and x = 1. Using x + y = 1 and z = 0 we get two critical points (1, 0, 0) and (0, 1, 0). The value of f at these points is 1, so the distance of these points from the origin is 1. If λ 1 then (12.1) and (12.2) imply that x = y. Plugging this into (12.4) we get 2x 2 = 1, so x = ± 2/2. If x = 2/2, we have y = x = 2/2 and we can use (12.5) to solve for z. This gives us a critical point ( 2/2, 2/2, 1 2). The value of f at this point is 1 + (1 z) If x = 2/2 we have y = x = 2/2 and we can use (12.5) to solve for z. This gives us a critical point ( 2/2, 2/2, 1 + 2). The value of f at this point is 1 + (1 + 2)

13 The smallest value of f at a critical point is 1. Thus, the points on C closest to the origin are (1, 0, 0) and (0, 1, 0) at a distance of 1. The largest value of f at a critical point is 1 + (1 + 2) 2. Thus, the point on C that is farthest from the origin is ( 2/2, 2/2, 1 + 2) at a distance of 1 + (1 + 2)

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