F.4 MATHS

1. C(n, n - 2) = 28
(Note: Actually C(n, n - 2) = C(n, 2))

(In general, C(n, n - r) = C(n, r))

n! / [ (n - 2)! 2! ] = 28
n*(n - 1)*(n - 2)! / [ (n - 2)! 2! ] = 28
n*(n - 1) / 2! = 28
n*(n - 1) = 56
n² - n - 56 = 0
(n - 8)(n + 7) = 0
n = 8 or n = -7 (rejected)
Thus, n = 8.

2. I assume you mean "Simplify (n+1)!/(n²+n)".
You missed the bracket in the denominator.

　(n + 1)! / (n² + n)
= (n + 1)! / [n(n + 1)]
= (n + 1)*n*(n - 1)! / [n(n + 1)]
= (n - 1)!


2014-03-13 10:55:45 補充：
其實。。。我係無瞓過～

因為有Ｄ野做緊。。。

上黎抖抖～

早晨，知識長！咁早瞓醒！

2014-03-13 11:16:21 補充：
保重! 保重!

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