Let f :   0 ,   ∞ → R be a differentiable function such that f ' x = 2 - f x x…

Let f: 0, R be a differentiable function such that f'x=2-fxxx(0, ) and f11. Then,
  1. limx0+ f 1x=1
  2. limx0+xf1x=2
  3. limx0+x2 f x=0
  4. fx2 for all x0, 2

Solution

Let y=fx
dydx+yx=2 (linear first order differential equation)

I.F.=edxx=elogex=x, (I.F.=Integrating factor)

 y·x=2xdx+c
 yx=x2+c

fx=x+cx;
As f11 c0
f'x=1-cx2, c 0
limx0+f'1x=limx0+1-cx2=1
limx0+xf1x=limx0+x1x+cx=limx0+1+cx2=1
limx0+x2f'x=limx0+x21-cx2=limx0+x2-c=-c0 as c0
fx=x+cx, c0
 limx0+fx= function is not bounded in 0, 2.

Asked in: JEE Advanced 2016 (Paper 1)

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