7 Let x be the length and width of a rectangular box with a fixed height h 3 A find total surface area S including the top and the bottom in terms of x and the volume V in terms of x Solve x in terms of S then find the formula V in terms of B find dV in terms of dS approximate the increase in volume using dV when S changes from S 320 m to S 321 m 8 Given that x x t y y t are functions oft and that xy y x y Take derivative or d dt on both sides then solve dy dt in terms of x y and dx dt Find dy dt if x 1 y 1 and dx dt 2