A kennel owner has 400 feet of fencing to enclose a rectangular dog exercise pen. Express the area of the exercise pen, A, as a function of one of its dimensions, x. A(x) = x(x - 200) A(x) = x(x - 200) A(x) = x(400 - x) A(x) = x(400 - x) A(x) = x(200 - x) A(x) = x(200 - x) A(x) = (200 - x)2