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JEE Advanced 2020Paper 1
Exam
JEE Advanced
Year
2020
Paper
Paper 1
Subject
Mathematics
Chapter
Application of Derivatives

JEE Advanced 2020 · Paper 1 · Mathematics · Application of Derivatives

For a polynomial g (x) with real coefficients, let m _ g denote the...

For a polynomial gg\left(x\right)withrealcoefficients,letwith real coefficients, let{m}_{g}denotethenumberofdistinctrealrootsofdenote the number of distinct real roots ofg\left(x\right).Suppose. SupposeS is the set of polynomials with real coefficients defined by S={(x2−1)2(a0+a1x+a2x2+a3x3):a0,a1,a2,a3∈R}S = \left\{{\left({x}^{2} - 1\right)}^{2}\left({a}_{0} + {a}_{1}x + {a}_{2}{x}^{2} + {a}_{3}{x}^{3}\right): {a}_{0}, {a}_{1}, {a}_{2}, {a}_{3} \in R\right\}. For a polynomial ff, let f′f ' and f″{f}^{″} denote its first and second order derivatives respectively. Then the minimum possible value of (mf′+mf″)\left(m{f}^{′} + m{f}^{″}\right), where f∈Sf \in S, is ____

Correct answer

0

Step-by-step solution

ff\left(x\right)isapolynomialinis a polynomial inxsuchthatsuch thatf\left(x\right) = \left\{{\left({x}^{2} - 1\right)}^{2}\left({a}_{0} + {a}_{1}x + {a}_{2}{x}^{2} + {a}_{3}{x}^{3}\right): {a}_{0}, {a}_{1}, {a}_{2}, {a}_{3} \in R\right\}</p><p>So,</p><p>So,{f}^{'}\left(x\right) = \left({x}^{2} - 1\right)\left[q\left(x\right)\right]wherewhereq(x)\left(x\right) is polynomial roots of f′(x)=−1,1{f}^{′}\left(x\right) = - 1, 1

So, by Rolle’s theorem, f(1)=0=f(−1)⇒f′(α)=0f\left(1\right) = 0 = f\left(- 1\right) ⇒ f ' \left(\alpha\right) = 0, where α∈(−1,1)\alpha \in \left(- 1, 1\right)

⇒⇒ minimum mf′=3m{f}^{′} = 3

Again by Rolle’s theorem, at least one real root lies in \left(- 1, \alpha\right)andatleastonerealrootliesinand at least one real root lies in\left(\alpha, 1\right)ofof{f}^{″}\left(x\right)</p><p>So,minimum</p><p>So, minimumm{f}^{″} = 2<br/><br />⇒Minimumpossiblevalueof(mf′+mf″)=5Minimum possible value of \left(m{f}^{′} + m{f}^{″}\right) = 5

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