<image>(2013 Autumn•End of Semester Exam in Xiangcheng District) As shown in the figure, cut off four equally sized small squares from the corners of a square piece of paper, and then fold it to form an open-top rectangular box. Let the side length of the square piece of paper be a and the height of the open-top rectangular box be h.

(1) If a = 18cm and h = 4cm, then the base area of the open-top rectangular box is ____;
(2) Express the volume V of the open-top rectangular box in terms of a and h: V = ____;
(3) If a = 18cm, explore whether the volume V of the open-top rectangular box increases as h increases. Please provide examples to illustrate; the maximum volume of this open-top rectangular box is ____.