JEE Advanced · Mathematics · 25. AOD
Let \(p(x)\) be a polynomial of degree 4 having extremum at \(x=1,2\) and \(\lim _{x \rightarrow 0}\left[1+\frac{p(x)}{x^2}\right]=2\). Then, the value of \(p(2)\) is
- A \(0\)
- B 5
- C 9
- D 10
Answer & Solution
Correct Answer
(A) \(0\)
Step-by-step Solution
Detailed explanation
Let \(p(x)=a x^4+b x^3+c x^2+d x+e\)
\[
\Rightarrow \quad p^{\prime}(x)=4 a x^3+3 b x^2+2 c x+d
\]
\[
\therefore \quad p^{\prime}(1)=4 a+3 b+2 c+d=0
\]
and \(p^{\prime}(2)=32 a+12 b+4 c+d=0\)
Since, \(\lim _{x \rightarrow 0}\left(1+\frac{p(x)}{x^2}\right)=2 \quad\) [given]
\[
\lim _{x \rightarrow 0} \frac{a x^4+b x^3+(c+1) x^2+d x+e}{x^2}=2
\]
\[
\begin{array}{rlrl}
\Rightarrow & & x+1=2, d & =0, e=0 \\
\Rightarrow & c & =1
\end{array}
\]
From Eqs. (i) and (ii)
\[
\begin{array}{rlrl}
& & 4 a+3 b=-2 & \text { and } 32 a+12 b=-4 \\
\Rightarrow & & a=\frac{1}{4} \text { and } b=-1 \\
\therefore & & p(x)=\frac{x^4}{4}-x^3+x^2 \\
\Rightarrow & & p(2)=\frac{16}{4}-8+4
\end{array}
\]
\[
\Rightarrow \quad p^{\prime}(x)=4 a x^3+3 b x^2+2 c x+d
\]
\[
\therefore \quad p^{\prime}(1)=4 a+3 b+2 c+d=0
\]
and \(p^{\prime}(2)=32 a+12 b+4 c+d=0\)
Since, \(\lim _{x \rightarrow 0}\left(1+\frac{p(x)}{x^2}\right)=2 \quad\) [given]
\[
\lim _{x \rightarrow 0} \frac{a x^4+b x^3+(c+1) x^2+d x+e}{x^2}=2
\]
\[
\begin{array}{rlrl}
\Rightarrow & & x+1=2, d & =0, e=0 \\
\Rightarrow & c & =1
\end{array}
\]
From Eqs. (i) and (ii)
\[
\begin{array}{rlrl}
& & 4 a+3 b=-2 & \text { and } 32 a+12 b=-4 \\
\Rightarrow & & a=\frac{1}{4} \text { and } b=-1 \\
\therefore & & p(x)=\frac{x^4}{4}-x^3+x^2 \\
\Rightarrow & & p(2)=\frac{16}{4}-8+4
\end{array}
\]
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