WAEC - Further Mathematics (2019 - No. 5)
If \(^nC_2\) = 15, find the value of n
8
7
6
5
説明
\(^nC_2\) = 15
\(\frac{n!}{(n - 3)! 2!}\) = 15
\(\frac{n(n - 1)(n - 2)!}{(n - 2)!2}\) = 15
n\(^2\) - n = 30
n\(^2\) - n - 30 = 0
(n - 6)(n + 5) = 0
n = 6 or n = -5