18) Stan wants a rectangular field to be enclosed on four sides with a fence on a plot of land he 18) owns. Fencing costs $8 per foot for two opposite sides, and $5 per foot for the other two sides. Stan calculated that the dimensions of the field of area 760 ft2 that would be the cheapest to enclose is approximately 34.9ft @ $5 per foot by 21.8 ft @ $8 per foot. Using calculus, show the calculations below that Stan would have had to do in order to arrive at these dimensions.