Let f : R → 0 , 1 , be a continuous function then, which of the following function(s), has(have) the…

Let f:R0,1 , be a continuous function then, which of the following function(s), has(have) the values zero at some point in the interval, (0,1)?
  1. x-0π2-xftcostdt
  2. x9-fx
  3. ex-0xftsintdt
  4. fx+0π2ftsintdt

Solution

For option (i),

Let  Mx=x-0π2-xft·cost dt

 M0=0-0π2ft.cost dt<0

Also,  M1=1-0π2-1ft.cost dt>0

   M0. M1<0     (so M(x) can be zero)

For option (ii),

Let,   kx=x9-fx

Now,  k0=-f0<0, as  f0, 1

Also,  k1=1-f1>0   As f0,1

⇒     k0. k1<0     (so k(x) can be zero)

For option (iii),

Let  gx=ex-0xftsintdt

 g'x=ex-fx·sinx>0  x0, 1    (as f(x)·sin x , is less than 1)

gx is strictly increasing function.

 gx>1 x0, 1

Also,  g0=1, (so g(x) is, never zero)

For option (iv),

Let, Tx=fx+0π2ft.sint dt

   Tx>0 x0, 1   As f0, 1     (so T(x) can not be zero)

 

Asked in: JEE Advanced 2017 (Paper 1)

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