7.) A farmer has $5200 to spend on fencing to make a pen to hold animals along a river. The fencing costs $6.50/m. The farmer wants to build a rectangular pen using the river as one of the sides, as shown in the diagram below. His daughter, a grade 10 math student, suggests that they build a pen in the shape of a right-angled triangle using the river as the hypotenuse. Determine using mathematical calculations the dimensions of largest area pen that can made using each shape and state which shape the farmer should use, if he wants the pen to have the maximum area?