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JEE Advanced 2009 · Application of Derivatives

Let p(x)p(x) be a polynomial of degree 4 having extremum at x=1,2x = 1, 2 and lim⁡x→0(1+p(x)x2)=2\lim_{x \to 0}\left(1 + \frac{p(x)}{x^{2}}\right) = 2. Then the value of p(2)p(2) is

Answer: 99

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Written solution

Let p(x) be a polynomial of° 4 having extremum at x = 1, 2 and lim_(x → 0)(1 + p(x)/x²) = 2. Then the value of p(2) is

p(x) degree 4,  extrema at x=1, 2p(x) \text{ degree } 4,\ \text{ extrema at } x=1,\ 2

lim⁡x→0(1+p(x)x2)=2\lim_{x\to0}\left(1+\dfrac{p(x)}{x^2}\right)=2

To find: p(2)\text{To find: } p(2)

use: limit condition, then p′(1)=p′(2)=0\text{use: limit condition, then } p'(1)=p'(2)=0

p(x)=ax4+bx3+cx2+dx+ep(x)=ax^4+bx^3+cx^2+dx+e lim⁡x→0(1+p(x)x2)=2⇒p(x)x2→1\lim_{x\to0}\left(1+\dfrac{p(x)}{x^2}\right)=2 \Rightarrow \dfrac{p(x)}{x^2}\to 1 p(x)x2=ax2+bx+c+dx+ex2→1\dfrac{p(x)}{x^2}=ax^2+bx+c+\dfrac dx+\dfrac e{x^2} \to 1 ⇒d=e=0,c=1\Rightarrow d=e=0,\quad c=1 p(x)=ax4+bx3+x2p(x)=ax^4+bx^3+x^2 1. p′(1)=p′(2)=01.\ p'(1)=p'(2)=0 2. solve for a,b2.\ \text{solve for } a, b 3. p(2)3.\ p(2) p′(x)=4ax3+3bx2+2xp'(x)=4ax^3+3bx^2+2x p′(1)=0: 4a+3b+2=0p'(1)=0:\ 4a+3b+2=0 p′(2)=0: 8a+3b+1=0p'(2)=0:\ 8a+3b+1=0 4a=1⇒a=144a=1 \Rightarrow a=\tfrac14 3b=−3⇒b=−13b=-3 \Rightarrow b=-1 p(x)=14x4−x3+x2p(x)=\tfrac14 x^4 - x^3 + x^2 p′(x)=x3−3x2+2x=x(x−1)(x−2)p'(x)=x^3-3x^2+2x=x(x-1)(x-2) p(2)=14(16)−8+4=4−8+4=0p(2)=\tfrac14(16)-8+4 = 4-8+4 = 0 p(2)=0\boxed{p(2)=0}

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