pls show all proofs and calculations

3. You need to move a set of n boxes from one place to another, where each box has positive weight w. You move boxes one at a time, and you tire as you move them, so the stress that moving each box causes you is the product of its weight and the square of how many boxes you have moved so far, including the current box. So your total stress for the boxes, moved one at a time in the chosen order, is You have chosen to move these boxes in nonincreasing order of weight (with the heaviest box first). Prove that this order will minimize the total stress