Medium INTEGER +4 / -1 PYQ · JEE Mains 2023

Let $f(x)=\sum_\limits{k=1}^{10} k x^{k}, x \in \mathbb{R}$. If $2 f(2)+f^{\prime}(2)=119(2)^{\mathrm{n}}+1$ then $\mathrm{n}$ is equal to ___________

Answer (integer) 10

Solution

Given, $f(x)=\sum_\limits{k=1}^{10} k x^{k}$ <br/><br/>$$ \begin{aligned} & f(x)=x+2 x^2+\ldots \ldots \ldots+10 x^{10} \\\\ & f(x) . x=x^2+2 x^3+\ldots \ldots \ldots+9 x^{10}+10 x^{11} \\\\ & f(x)(1-x)=x+x^2+x^3+\ldots \ldots \ldots+x^{10}-10 x^{11} \\\\ & \therefore f(x)=\frac{x\left(1-x^{10}\right)}{(1-x)^2}-\frac{10 x^{11}}{(1-x)} \end{aligned} $$ <br/><br/>$\Rightarrow$ $$ f(x)=\frac{x-x^{11}-10 x^{11}+10 x^{12}}{(1-x)^2} = \frac{10 x^{12}-11 x^{11}+x}{(1-x)^2} $$ <br/><br/>So, <br/><br/>$$ \begin{aligned} & f(2)=2\left(1-2^{10}\right)+10 \cdot 2^{11} \\\\ & =2+18 \cdot 2^{10} \end{aligned} $$ <br/><br/>$$f'\left( x \right) = {{{{\left( {1 - x} \right)}^2}\left( {120{x^{11}} - 121{x^{10}} + 1} \right) + 2\left( {1 - x} \right)\left( {10{x^{12}} - {{11.x}^{11}} + 2} \right)} \over {{{\left( {1 - x} \right)}^4}}}$$ <br/><br/>So, <br/><br/>$$f'\left( x \right) = {{1\left( {{{120.2}^{11}} - {{121.2}^{10}} + 1} \right) + 2\left( { - 1} \right)\left( {{{10.2}^{12}} - {{11.2}^{11}} + 2} \right)} \over {{{\left( { - 1} \right)}^4}}}$$ <br/><br/>= ${2^{10}}\left( {83} \right) - 3$ <br/><br/>Hence $2 f(2)+f^{\prime}(2)=119.2^{10}+1$ <br/><br/>$\Rightarrow \text { So, } \mathrm{n}=10$

About this question

Subject: Mathematics · Chapter: Differentiation · Topic: Derivatives of Standard Functions

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