If the Coefficient of $x^{30}$ in the expansion of $\left(1+\frac{1}{x}\right)^6\left(1+x^2\right)^7\left(1-x^3\right)^8 ; x \neq 0$ is $\alpha$, then $|\alpha|$ equals ___________.
Answer (integer)
678
Solution
$\begin{aligned} & \text { Coefficient of } x^{30} \text { in } \frac{(1+x)^6\left(1+x^2\right)^7\left(1-x^3\right)^8}{x^6} \\\\ & \Rightarrow \text { Coefficient of } x^{36} \text { in }(1+x)^6\left(1+x^2\right)^7\left(1-x^3\right)^8 \\\\ & \Rightarrow \text { General term }={ }^6 C_{r_1}{ }^7 C_{r_2}{ }^8 C_{r_3}(-1)^{r_3} x^{r_1+2 r_2+3 r_3} \\\\ & \Rightarrow r_1+2 r_2+3 r_3=36\end{aligned}$
<br/><br/>$$
\text { Case-I : } \begin{array}{|c|c|c|}
\hline \mathrm{r}_1 & \mathrm{r}_2 & \mathrm{r}_3 \\
\hline 0 & 6 & 8 \\
\hline 2 & 5 & 8 \\
\hline 4 & 4 & 8 \\
\hline 6 & 3 & 8 \\
\hline
\end{array}
$$
<br/><br/>$r_1+2 r_2=12 \quad\left(\right.$ Taking $\left.r_3=8\right)$
<br/><br/>$$
\begin{aligned}
&\text { Case-II :}\\\\
&\begin{array}{|l|l|l|}
\hline r_1 & r_2 & r_3 \\
\hline 1 & 7 & 7 \\
\hline 3 & 6 & 7 \\
\hline 5 & 5 & 7 \\
\hline
\end{array}
\end{aligned}
$$
<br/><br/>$r_1+2 r_2=15\left(\right.$ Taking $\left.r_3=7\right)$
<br/><br/>$$
\begin{aligned}
&\text { Case-III : }\\\\
&\begin{array}{|l|l|l|}
\hline r_1 & r_2 & r_3 \\
\hline 4 & 7 & 6 \\
\hline 6 & 6 & 6 \\
\hline
\end{array}
\end{aligned}
$$
<br/><br/>$\begin{aligned} & \text { Coefficient}=7+(15 \times 21)+(15 \times 35)+(35) \\\\ & -(6 \times 8)-(20 \times 7 \times 8)-(6 \times 21 \times 8)+(15 \times 28) \\\\ & +(7 \times 28)=-678=\alpha \\\\ & |\alpha|=678\end{aligned}$
About this question
Subject: Mathematics · Chapter: Binomial Theorem · Topic: Binomial Expansion
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