MIT OpenCourseWare http://ocw.mit.edu 18.02 Multivariable CalculusFall 2007 For information about citing these materials or our Terms of Use, visit: http://ocw.mit.edu/terms.18.02 Practice Exam 2 B Problem 1. Let f(x, y) = x2y2 − x. a) (5) Find �f at (2, 1) b) (5) Write the equation for the tangent plane to the graph of f at (2, 1, 2). c) (5) Use a linear approximation to find the approximate value of f(1.9, 1.1). d) (5) Find the directional derivative of f at (2, 1) in the direction of −ˆı +ˆ�. Problem 2. (10) On the contour plot below, mark the portion of the level curve f = 2000 on �f which � 0. �y Problem 3. a) (10) Find the critical points of w = −3x 2 − 4xy − y 2 − 12y + 16x and say what type each critical point is. b) (10) Find the point of the first quadrant x � 0, y � 0 at which w is largest. Justify your answer. Problem 4. Let u = y/x, v = x2 + y2 , w = w(u, v). a) (10) Express the partial derivatives wx and wy in terms of wu and wv (and x and y). b) (7) Express xwx + ywy in terms of wu and wv. Write the coefficients as functions of u and v. c) (3) Find xwx + ywy in case w = v5 . Problem 5. a) (10) Find the Lagrange multiplier equations for the point of the surface x 4 + y 4 + z 4 + xy + yz + zx = 6 at which x is largest. (Do not solve.) b) (5) Given that x is largest at the point (x0, y0, z0), find the equation for the tangent plane to the surface at that point. Problem 6. Suppose that x 2 + y 3 − z 4 = 1 and z 3 + zx + xy = 3. a) (8) Take the total differential of each of these equations. b) (7) The two surfaces in part (a) intersect in a curve along which y is a function of x. Find dy/dx at (x, y, z) = (1, 1,
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