Answer:
A(x) = x(200-x)
Step-by-step explanation:
400 feet of fencing to enclose a rectangular dog exercise pen means the perimeter of rectangle is 400 feet
If x is length and y is breadth. Perimeter is given by :
P = 2(x+y)
400 = 2(x+y)
⇒ (x+y) = 200 .....(1)
y = 200-x
Area of a rectangle is given by :
A = xy
A(x) = x(200-x)
Hence, the area as a function of length of rectangle is x(200-x).