In the expansion of $$(1+x)\left(1-x^2\right)\left(1+\frac{3}{x}+\frac{3}{x^2}+\frac{1}{x^3}\right)^5, x \neq 0$$, the sum of the coefficients of $x^3$ and $x^{-13}$ is equal to __________.
Answer (integer)
118
Solution
<p>$$\begin{aligned}
& (1+x)\left(1-x^2\right)\left(1+\frac{3}{x}+\frac{3}{x^2}+\frac{1}{x^3}\right)^5 \\
& =(1+x)\left(1-x^2\right)\left(\left(1+\frac{1}{x}\right)^3\right)^5 \\
& =\frac{(1+x)^2(1-x)(1+x)^{15}}{x^{15}} \\
& =\frac{(1+x)^{17}-x(1+x)^{17}}{x^{15}}
\end{aligned}$$</p>
<p>$=\operatorname{coeff}\left(\mathrm{x}^3\right)$ in the expansion $\approx \operatorname{coeff}\left(\mathrm{x}^{18}\right)$ in</p>
<p>$$\begin{aligned}
& (1+x)^{17}-x(1+x)^{17} \\
& =0-1 \\
& =-1
\end{aligned}$$</p>
<p>$\operatorname{coeff}\left(\mathrm{x}^{-13}\right)$ in the expansion $\approx \operatorname{coeff}\left(\mathrm{x}^2\right)$ in</p>
<p>$$\begin{aligned}
& (1+x)^{17}-x(1+x)^{17} \\
& =\left(\begin{array}{c}
17 \\
2
\end{array}\right)-\left(\begin{array}{c}
17 \\
1
\end{array}\right) \\
& =17 \times 8-17 \\
& =17 \times 7 \\
& =119
\end{aligned}$$</p>
<p>Hence Answer $=119-1=118$</p>
About this question
Subject: Mathematics · Chapter: Binomial Theorem · Topic: Binomial Expansion
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