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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 coefficient of $x^3$ and $x^{-13}$ is equal to
$118$
$116$
$115$
$117$
Solution
$\begin{aligned} & (1+x)\left(1-x^2\right)\left(1+\frac{3}{x}+\frac{3}{x^2}+\frac{1}{x^3}\right)^5 \\ & =(1+x)\left(1-x^2\right)\left(\left(1+\frac{1}{x}\right)^3\right)^5 \\ & =\frac{(1+x)^2(1-x)(1+x)^{15}}{x^{15}} \\ & =\frac{(1+x)^{17}-x(1+x)^{17}}{x^{15}} \\ & =\operatorname{coeff}\left(x^3\right) \text { in the expansion } \approx \operatorname{coeff}\left(x^{18}\right) \text { in } \\ & (1+x)^{17}-x(1+x)^{17} \\ & =0-1 \\ & =-1 \\ & \text { coeff }\left(x^{-13}\right) \text { in the expansion } \approx \operatorname{coeff}\left(x^2\right) \quad \text { in }\end{aligned}$
$(1+x)^{17}-x(1+x)^{17}$
$=0-1$
$=-1$
$\operatorname{coeff}\left(\mathrm{x}^{-13}\right)$ in the expansion $\approx \operatorname{coeff}\left(\mathrm{x}^2\right)$ in
$(1+x)^{17}-x(1+x)^{17}$
$=\left(\begin{array}{c}17 \\ 2\end{array}\right)-\left(\begin{array}{c}17 \\ 1\end{array}\right)$
$=17 \times 8-17$
$=17 \times 7$
$=119$
Hence Answer $=119-1=118$