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JEE Advanced 2016Paper 1
Exam
JEE Advanced
Year
2016
Paper
Paper 1
Subject
Mathematics
Chapter
Differential Equations

JEE Advanced 2016 · Paper 1 · Mathematics · Differential Equations

Let f: (0, ∞) → R be a differentiable function such that f ^ ' (x)...

Let f:(0,∞)→Rf: \left(0, \infty\right) \to R be a differentiable function such that f′(x)=2−f(x)x∀x∈(0,∞){f}^{'}\left(x\right) = 2 - \dfrac{f\left(x\right)}{x} \forall x \in ( 0, \infty ) and f(1)≠1f\left(1\right) \ne 1. Then,

Options

  1. Alim⁡x→0+f′(1x)=1\lim_{x \to {0}^{+}} {f}^{′}\left(\dfrac{1}{x}\right) = 1
  2. Blim⁡x→0+xf(1x)=2\lim_{x \to {0}^{+}} xf\left(\dfrac{1}{x}\right) = 2
  3. Clim⁡x→0+x2f′(x)=0\lim_{x \to {0}^{+}} {x}^{2}{f}^{′}\left(x\right) = 0
  4. D∣f(x)∣≤2\left|f\left(x\right)\right| \le 2 for all x∈(0,2)x \in \left(0, 2\right)

Correct answer

A. lim⁡x→0+f′(1x)=1\lim_{x \to {0}^{+}} {f}^{′}\left(\dfrac{1}{x}\right) = 1

Step-by-step solution

Let y=f(x)y = f\left(x\right)
dydx+yx=2\dfrac{dy}{dx} + \dfrac{y}{x} = 2 (linear first order differential equation)

I.F.=e∫dxx=elog⁡e(x)=xI . F . = {e}^{∫\dfrac{dx}{x}} = {e}^{\log_{e}\left(x\right)} = x, (I.F.=Integrating factor)

∴y⋅x=2∫xdx+c∴ y \cdot x = 2 ∫ xdx + c
∴yx=x2+c∴ yx = {x}^{2} + c

⇒f(x)=x+cx;⇒ f\left(x\right) = x + \dfrac{c}{x};
As f(1)≠1⇒c≠0f\left(1\right) \ne 1 ⇒ c \ne 0
⇒f′(x)=1−cx2,c≠0⇒ {f}^{'}\left(x\right) = 1 - \dfrac{c}{{x}^{2}}, c \ne 0
lim⁡x→0+f′(1x)=lim⁡x→0+(1−cx2)=1\lim_{x \to {0}^{+}} {f}^{'}\left(\dfrac{1}{x}\right) = \lim_{x \to {0}^{+}} \left(1 - c{x}^{2}\right) = 1
lim⁡x→0+xf(1x)=lim⁡x→0+x(1x+cx)=lim⁡x→0+(1+cx2)=1\lim_{x \to {0}^{+}} xf\left(\dfrac{1}{x}\right) = \lim_{x \to {0}^{+}} x\left(\dfrac{1}{x} + cx\right) = \lim_{x \to {0}^{+}} \left(1 + c{x}^{2}\right) = 1
lim⁡x→0+x2f′(x)=lim⁡x→0+x2(1−cx2)=lim⁡x→0+(x2−c)=−c≠0\lim_{x \to {0}^{+}} {x}^{2}{f}^{'}\left(x\right) = \lim_{x \to {0}^{+}} {x}^{2}\left(1 - \dfrac{c}{{x}^{2}}\right) = \lim_{x \to {0}^{+}} \left({x}^{2} - c\right) = - c \ne 0 as c≠0c \ne 0
f(x)=x+cx,c≠0f\left(x\right) = x + \dfrac{c}{x}, c \ne 0
∴lim⁡x→0+f(x)=∞∴ \lim_{x \to {0}^{+}} f\left(x\right) = \infty ⇒⇒function is not bounded in \left(0, 2\right)$.

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