If c(n, n-1) = 36, c (n, r) = 84 and c(n, n + 1) = 126, then

n = 9

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n = 10

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n = 6

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n = 3

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Solution

The correct options are
A n = 9
D n = 3
$\frac{^{n} C_{r - 1}}{^{n} C_{r}} = \frac{r}{n - r + 1} = \frac{3}{7} - - - - - (1) \frac{^{n} C_{r}}{^{n} C_{r + 1}} = \frac{r + 1}{n - r} = \frac{2}{3} - - - - - (2)$
From (1) and (2), n = 9, r=3

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