Q35 / 54 · JEE Advanced 2020 (Paper 1) · Mathematics

For a polynomial gx with real coefficients, let mg denote the number of distinct real roots of gx. Suppose S is the set of polynomials with real coefficients defined by S=x2−12a0+a1x+a2x2+a3x3 :a0, a1, a2, a3∈R. For a polynomial f, let f' and f″ denote its first and second order derivatives respectively. Then the minimum possible value of mf′ +mf″ , where f∈S, is ____

Correct answer: 5

Solution

fx is a polynomial in x such that fx=x2−12 a0+a1x+a2x2+a3x3 : a0, a1, a2, a3∈R

So, f'x=x2−1 qx where qx is polynomial roots of f′x=−1, 1

So, by Rolle’s theorem, f1=0=f−1⇒f'α=0, where α∈−1, 1

⇒ minimum mf′ =3

Again by Rolle’s theorem, at least one real root lies in −1, α and at least one real root lies in α, 1 of f″x

So, minimum mf″ =2
⇒ Minimum possible value of mf′ +mf″ =5

Hence, the correct answer is 5.

Relevant Questions