If the coefficient of $(2 r + 4)^{t h}$ and $(r - 2)^{t h}$ terms in the expansion of $(1 + x)^{18}$ are equal, then r =

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Solution

The correct option is D 6
$^{18} C_{2 r + 3} =^{18} C_{r - 3} \Rightarrow 2 r + 3 + r - 3 = 18 \Rightarrow r = 6$

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If the coefficient of $(2 r + 4)^{t h}$ and $(r - 2)^{t h}$ terms in the expansion of ( $1 + x)^{18}$ are equal, then r =

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