(a) Derivative in velocity direction: Find the derivative of /(a, y, 2) = xyz in the direction of the velocity vector of the helix r(t) = (cos 3t) i + (sin 3t) j + 3/ k . at t = 7/3. (b) Maximum in closed first quadrant: Find the maximum values of f(x, y) = 6xye-(27+3y) in the closed first quadrant (includes the non-negative axes) . (c) Minimizing cost of a box: A closed rectangular box is to have volume V cm3. The cost of the material used in the box is a cents/cm for top and bottom, b cents/ cm for front and back, and c cents/cm for the remaining sides. What dimensions minimize the total cost of materials