A rectangle is constructed with its base on the diameter of a semicircle with radius 18 and with its two other vertices on the semicircle. What are the dimensions of the rectangle with maximum area? Let A be the area of the rectangle. What is the objective function in terms of the base of the rectangle, x? A = (Type an expression.) The interval of interest of the objective function is Simplify your answer. Type your answer in interval notation.) The rectangle with maximum area has base and height (Type exact answers, using radicals as needed.)