If the coefficient of (2r + 4)th term is equal to the coefficient of (r - 2)th term in the expansion of $(1 + x)^{18}$ then r $=$

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Solution

The correct option is C 6
Coefficient of (2r + 4)th term $=^{18} C_{2 r + 3}$
Coefficient of (r - 2)th term $=^{18} C_{r - 3}$
Coefficient of (2r + 4)th term
$=$ coefficient of (r - 2)th term
$\Rightarrow^{18} C_{2 r + 3} =^{18} C_{r - 3}$
$\Rightarrow 2 r + 3 + r - 3 = 18$
$\Rightarrow 3 r = 18 \Rightarrow r = 6$

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