PrepRung

JEE Advanced 2021 Paper 1 · Mathematics · Differentiation and Application of Derivatives · hard

The number of real solutions of the equation e^x=2x^2 is ____.

Numerical answer

The number of real solutions of the equation ex=2x2e^{x}=2x^{2} is ____.
Show answer and solution

Answer: 3

Roots of ex=2x2e^x=2x^2 are not at x=0x=0 (left side is 11, right side 00), so divide by x2x^2 and study h(x)=exx2=2h(x)=\dfrac{e^x}{x^2}=2. Its derivative is h′(x)=ex(x−2)x3h'(x)=\dfrac{e^x(x-2)}{x^3}, which is positive for x<0x<0 (numerator and x3x^3 both negative), so on (−∞,0)(-\infty,0) the function hh increases from 00 to +∞+\infty as xx goes from −∞-\infty to 0−0^-, crossing 22 exactly once. For x>0x>0, h′<0h'<0 on (0,2)(0,2) and h′>0h'>0 on (2,∞)(2,\infty), with minimum h(2)=e24≈1.85<2h(2)=\dfrac{e^2}{4}\approx1.85<2 and h→∞h\to\infty at both ends of (0,∞)(0,\infty), so h=2h=2 has exactly two positive roots. The total is 1+2=31+2=3.
Report a mistake

Original question written in the JEE Advanced 2021 Paper 1 pattern and checked twice for the correct answer. Not an official question paper. How we write and check questions · Corrections policy · Report a mistake