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Question

If f:RR is a differentiable function such that f(x)>2f(x) for all xR, and f(0)=1, then

A
f(x) is increasing in(0,)
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B
f(x) is decreasing in(0,)
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C
f(x)>e2x in (0,)
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D
f(x)<e2x in (0,)
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Solution

The correct options are
A f(x) is increasing in(0,)
C f(x)>e2x in (0,)
f(x)2f(x)>0 xR (1)
Let y=f(x)
Then dydx2y>0
Integrating function I.F.=e2dx=e2x

Multiplying by e2x, we get
e2xdydx2ye2x>0
ddx(y.e2x)=ddx(f(x).e2x)>0

Suppose g(x)=f(x).e2x
Then g(x)>0 xR [From (1)]
g(x)=f(x).e2x is strictly increasing for all xR
Also, g(0)=1
g(x)>g(0)=1 for all x>0
f(x).e2x>1 x(0,)
Hence f(x)>e2x x(0,)
As f(x)>2f(x)f(x)>2e2x>2 x(0,)
f(x) is strictly increasing in (0,)

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