The coefficient of $x^{n}$ in the expansion of $(1 + 2 x + 3 x^{2} + . . .)^{1 / 2}$ is equal to:

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-n

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None

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Solution

The correct option is A
1

1 + 2x + $3 x^{2} + . . . . = (1 - x)^{- 2}$

$\Rightarrow$ $(1 + 2 x + 3 x^{2} + . . . . .)^{1 / 2} = (1 - x)^{- 1}$

$∴$ coefficient of $x^{n} = 1$ .

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