2) а) (n +1)! /(n -1)! = 1*2**3*...*(n-1)* n* (n+1) /(1*2*3*...*(n-1)) =
= n(n+1) = n² + n
б) (n-4)!/(n-2)! = (1*2*3*...*(n-4))/(1*2*3*...*(n-4)*(n-3)*(n-2)) =
= 1/(n-3)(n-2) = 1/(n² - 5n +6)
3) Cиз 9 элементов по 3 = 9!/(3!*6!) = (1*2*3*...*9)/(1*2*3*1*2*3*4*5*6)=
= 7*8*9/1*2*3 = 84