辅导案例-QUIZ 2

  • June 11, 2020

QUIZ 2 SAMPLE PROBLEMS (1) Calculate the partial derivatives fx and fy of f(x, y) = ln( x3−y3 1−y ). (2) Find the equation of the tangent plane of f(x, y) = x2 + y2 + 3x + 2y + 1 at the point (1, 2). (3) Let f(x, y) = x2y + sin(xy). Show that fxy = fyx. (4) Evaluate the following limit, or show that it does not exist. lim (x,y)→(0,0) x2y + y3 x2 + y2 . (5) Evaluate the following limit, or show that it does not exist. lim (x,y)→(0,0) 2×3 + 3y2 x2 + y2 . (6) The level curve ex−y + x2 − y = 1 defines y as a function of x around the point P = (0, 0). Find dy/dx at P. (7) A particle moves on a surface whose equation is z = f(x, y) = (x− 1)2 + y2. Assume that the x and y coordinates of the particle at time t are given by x = 2 cos t and y = 2 sin t , where 0 ≤ t ≤ 2pi. Use the total derivative rule to show that the rate of change of the height of the particle above the xy plane is given by dz dt = 4 sin t. Hence find the maximum value of the height and the coordinates (x, y, z) of the particle when the maximum occurs. (8) The build up of minerals on the inner wall of a pipe has reduced its capacity to carry liquid. If the pipe has an elliptical cross section with semimajor axis 4 cm and semiminor axis 3 cm and the unwanted coating is 0.05 cm thick, use differentials to estimate the reduction in cross sectional area. (The area of an ellipse with semimajor axis a and semiminor axis b is piab.) 1 2 QUIZ 2 SAMPLE PROBLEMS Answers (1) fx = 3×2 x3−y3 and fy = x3+2y3−3y2 (1−y)(x3−y3) (2) z = 5x + 6y – 4 (3) fxy = 2x+ cos(xy)− xy sin(xy) (4) 0 (5) Limit does not exist. (6) 1/2 (7) Maximum z-value is 9, occuring at (−2, 0, 9) (when t = pi). (8) 1.1cm2

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