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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