Tính: a) C 2 7; b) C 0 9 + c 9 9; c) c) C 3 15 - C 3 14.

a) \(C_7^2 = \frac{{7!}}{{2!\left( {7 - 2} \right)!}} = \frac{{7.6.5.4.3.2.1}}{{2.1.5.4.3.2.1}} = \frac{{7.6}}{{2.1}} = 21.\)

b) \(C_9^0 + C_9^9 = \frac{{9!}}{{0!.(9 - 0)!}} + \frac{{9!}}{{9!.(9 - 9)!}} = 1 + 1 = 2\).

c) \(C_{15}^3 - C_{14}^3 = \frac{{15!}}{{3!.\left( {15 - 3} \right)!}} - \frac{{14!}}{{3!.\left( {14 - 3} \right)!}} = \frac{{15!}}{{3!.12!}} - \frac{{14!}}{{3!.11!}} = \frac{{15.14.13}}{{3.2.1}} - \frac{{14.13.12}}{{3.2.1}} = 91\).