If the second, third and fourth terms in the expansion of $(x+y)^n$ are 135, 30 and $\frac{10}{3}$, respectively, then $6\left(n^3+x^2+y\right)$ is equal to __________.
Answer (integer)
806
Solution
<p>$$\begin{aligned}
& T_2={ }^n C_1 y^1 \cdot x^{n-1}=135 \\
& T_3={ }^n C_2 y^2 \cdot x^{n-2}=30 \\
& T_4={ }^n C_3 y^3 x^{n-3}=\frac{10}{3}
\end{aligned}$$</p>
<p>$$\begin{aligned}
\Rightarrow \frac{135}{30} & =\left(\frac{x}{y}\right) \frac{n \cdot 2}{n(n-1)}=\left(\frac{2}{n-1}\right)\left(\frac{x}{y}\right) \quad \text{... (i)}\\
\frac{30}{\frac{10}{3}} & =\frac{n(n-1)}{2} \frac{3!}{n(n-1)(n-2)}\left(\frac{x}{y}\right) \\
9 & =\left(\frac{3}{n-2}\right)\left(\frac{x}{y}\right)
\end{aligned}$$</p>
<p>$$\begin{aligned}
\Rightarrow & 3(n-2)=\frac{135}{60}(n-1) \Rightarrow n=5 \\
\Rightarrow & x=9 y \quad \text{.... (i)}\\
& y \cdot x^4=27 \Rightarrow \frac{x}{9} \cdot x^4=3^3 \\
\Rightarrow & x^5=3^5 \Rightarrow x=3 y=\frac{1}{3} \\
\Rightarrow & 6\left(5^3+3^2+\frac{1}{3}\right)=6\left(125+9+\frac{1}{3}\right)
\end{aligned}$$</p>
<p>$=6(134)+2=806$</p>
About this question
Subject: Mathematics · Chapter: Binomial Theorem · Topic: Binomial Expansion
This question is part of PrepWiser's free JEE Main question bank. 193 more solved questions on Binomial Theorem are available — start with the harder ones if your accuracy is >70%.