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WAEC Further Mathematics 2019 Paper

Question : 5

Total: 39

{}^nC_2

= 15, find the value of n

Solution:

${}^nC_2$ = 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

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