Q.

A differentiable function f:RR satisfies the functional equation f(x).f(y)+f(x+y)=exf(y)+eyf(x)+xy   x,yR. If f'(0)=0 and f(0)=0 then which of the following statements is/are correct?

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a

There exists atleast one horizontal tangents to the curve y=f(x) in  (1,1)

b

limx0f(x)x2=12

c

xx2f'(t)f(t)dt0|x|1

d

F(x2)>F(x1),  x2>x1,  F(x)=f'(x)f(x)

answer is B, C, D.

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Detailed Solution

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f(x).f(y)+f(x+y)=exf(y)+eyf(x)+xyf(x).f'(y)+f'(x+y)=exf'(y)+eyf(x)+xPut   y=0f(x).f'(0)+f'(x)=exf'(0)+f(x)+xf'(x)f(x)=xf(x)=exx1(A)limx0f(x)x2=limx0exx1x2=12(B)xx2tdt=(t22)xx2=x4x220  |x|1

(C)  F(x)=f'(x)f(x)=(ex1)(exx1)=x   whichisincreasing  function  so F(x2)>F(x1)  if  x2>x1(D)  Now   f'(x)=ex1andf''(x)=ex>0  xRf'(x)is  increasing  function  or  Rf'(1)=ve  and  f'(1)=+vef'(c)=0  has  exactly  are  root  in(1,1).i.e.  onehorizontaltangent.  ln(1,1)

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A differentiable function f:R→R satisfies the functional equation f(x).f(y)+f(x+y)=exf(y)+eyf(x)+xy   ∀ x,y∈R. If f'(0)=0 and f(0)=0 then which of the following statements is/are correct?