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16. Find the directional derivative of f(x, y) = (x - 1)y^2e^{xy} at (0, 1) toward the point (-1, 3). 17. Find the directional derivative of f(x, y) = Ax^2 + 2Bxy + Cy^2 at (a, b) toward (b, a) (a) if a > b; (b) if a < b. 18. Find the directional derivative of f(x, y, z) = z ln(x/y) at (1, 1, 2) toward the point (2, 2, 1). 19. Find the directional derivative of f(x, y, z) = xe^{y^2-z^2} at (1, 2, -2) in the direction of increasing t along the path r(t) = ti + 2 cos(t - 1)j - 2e^{t-1}k. 20. Find the directional derivative of f(x, y, z) = x^2 + yz at (1, -3, 2) in the direction of increasing t along the path r(t) = t^2i + 3tj + (1 - t^3)k. 21. Find the directional derivatives of f(x, y, z) = x^2 + 2xyz - yz^2 at (1, 1, 2) in the directions parallel to the line (x-1)/2 = y-1 = (z-2)/-3. 22. Find the directional derivatives of f(x, y, z) - e^x cos ?yz at (0, 1, 1/2) in the directions parallel to the line in which the planes x + y - z = 5 and 4x - y - z = 2 intersect. Exercises 23–26. Find the unit vector in the direction in which f increases most rapidly at P and give the rate of change of f in that direction; find the unit vector in the direction in which f decreases most rapidly at P and give the rate of change of f in that direction. 23. f(x, y) = y^2e^{2x}; P(0, 1).

          16. Find the directional derivative of f(x, y) = (x - 1)y^2e^{xy} at (0, 1) toward the point (-1, 3).
17. Find the directional derivative of f(x, y) = Ax^2 + 2Bxy + Cy^2 at (a, b) toward (b, a) (a) if a > b; (b) if a < b.
18. Find the directional derivative of f(x, y, z) = z ln(x/y) at (1, 1, 2) toward the point (2, 2, 1).
19. Find the directional derivative of f(x, y, z) = xe^{y^2-z^2} at (1, 2, -2) in the direction of increasing t along the path
r(t) = ti + 2 cos(t - 1)j - 2e^{t-1}k.
20. Find the directional derivative of f(x, y, z) = x^2 + yz at (1, -3, 2) in the direction of increasing t along the path
r(t) = t^2i + 3tj + (1 - t^3)k.
21. Find the directional derivatives of f(x, y, z) = x^2 + 2xyz - yz^2 at (1, 1, 2) in the directions parallel to the line
(x-1)/2 = y-1 = (z-2)/-3.
22. Find the directional derivatives of f(x, y, z) - e^x cos ?yz at (0, 1, 1/2) in the directions parallel to the line in which the planes x + y - z = 5 and 4x - y - z = 2 intersect.
Exercises 23–26. Find the unit vector in the direction in which f increases most rapidly at P and give the rate of change of f in that direction; find the unit vector in the direction in which f decreases most rapidly at P and give the rate of change of f in that direction.
23. f(x, y) = y^2e^{2x}; P(0, 1).

16. Find the directional derivative of f(x, y) = (x - 1)y^2e^xy at (0, 1) toward the point (-1, 3).
17. Find the directional derivative of f(x, y) = Ax^2 + 2Bxy + Cy^2 at (a, b) toward (b, a) (a) if a > b; (b) if a < b.
18. Find the directional derivative of f(x, y, z) = z ln(x/y) at (1, 1, 2) toward the point (2, 2, 1).
19. Find the directional derivative of f(x, y, z) = xe^y^2-z^2 at (1, 2, -2) in the direction of increasing t along the path
r(t) = ti + 2 cos(t - 1)j - 2e^t-1k.
20. Find the directional derivative of f(x, y, z) = x^2 + yz at (1, -3, 2) in the direction of increasing t along the path
r(t) = t^2i + 3tj + (1 - t^3)k.
21. Find the directional derivatives of f(x, y, z) = x^2 + 2xyz - yz^2 at (1, 1, 2) in the directions parallel to the line
(x-1)/2 = y-1 = (z-2)/-3.
22. Find the directional derivatives of f(x, y, z) - e^x cos ?yz at (0, 1, 1/2) in the directions parallel to the line in which the planes x + y - z = 5 and 4x - y - z = 2 intersect.
Exercises 23–26. Find the unit vector in the direction in which f increases most rapidly at P and give the rate of change of f in that direction; find the unit vector in the direction in which f decreases most rapidly at P and give the rate of change of f in that direction.
23. f(x, y) = y^2e^2x; P(0, 1).

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