Take your topic as an example. . . How many meanings do you mean by two nineties? . . What about the calculation? . There are formulas. . . This is a small number that changes c into p divided by p.

. . . . . It's hard to remember. . . Give you a thinking process. . . P 2 9 is nine, and two of them are in order. . C 2 9 is a two-ninth combination. P 2 9 is actually c 2 9 multiplied by 2 for sorting. . . My Chinese is not good. . . I don't know if I understand. . . Then. Know this. Formula deformation. . . P how to calculate this?

P n n is n times n- 1 times n-2 times 1.