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For \(x \in R, x \neq-1\), if
\((1+x)^{2016}+x(1+x)^{2015}+x^{2}(1+x)^{2014}+\ldots \ldots+x^{2016}=\sum_{i=0}^{2016} a_{i} \cdot x^{i}\), then \(a_{17}\) is equal to
(A) \(\frac{2016 !}{17 ! 1999 !}\)
(B) \(\frac{2016 !}{16 !}\)
(C) \(\frac{2017 !}{2000 !}\)
(D) \(\frac{2017 !}{17 ! 2000 !}\)

3 Answers

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Ans: (D)
Hint: \(a_{17}=\) coeff \(^{n}\) of \(x^{17}\)
$$
\begin{aligned}
&={ }^{2016} \mathrm{C}_{17}+{ }^{2015} \mathrm{C}_{16}+{ }^{2014} \mathrm{C}_{15}+\cdots+{ }^{1999} \mathrm{C}_{0} \\
&={ }^{2016} \mathrm{C}_{1999}+{ }^{2015} \mathrm{C}_{1999}+{ }^{2014} \mathrm{C}_{1999}+\cdots+{ }^{1999} \mathrm{C}_{1999} \\
&={ }^{2017} \mathrm{C}_{2000}=\frac{\mid 2017}{\mid 17 {2000!}{}}
\end{aligned}
$$
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