<image>As shown in the figure, the following series of figures are made up of matchsticks, where the nth figure is composed of n squares.

(1) The number of matchsticks in the 5th figure is ____;
(2) The number of matchsticks in the 8th figure is ____;
(3) The number of matchsticks in the 20th figure is ____;
(4) The number of matchsticks in the nth figure is ____.