Let f ⁡  :  1 2 1 ⟶ R (the set of all real numbers) be a positive, non-constant and…

Let f : 121R (the set of all real numbers) be a positive, non-constant and differentiable function such that f'x<2fx and f12=1. Then the value of 1 / 2 1 f x dx  lies in the interval:

  1. 2 e - 1 2 e
  2. e - 1 2 e - 1
  3. e - 1 2 e - 1
  4. 0 , e - 1 2

Solution

f'x<2fx
f'xfx<2

12xf'xfxdx=12x2dx

lnfx<2x-1

fx<e2x-1
0<121fxdx<121e2x-1dx

0<121fxdx<e2x-12121
0<121fxdx<e-12

Asked in: JEE Advanced 2013 (Paper 1)

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