原式 = [18/1 * 19/2 * 20/3 .* 36/19] / [20/1 * 21/2 * 22/3 ...* 36/17] =
[18/1 * 19/2 * 20/3 .* 36/19] * [1/20 * 2/21 * 3/22 *...* 17/36] =
36!/ 36!=
1