Question Details

Consider the following statements regarding the function f(x) = |ln x| − |x − 1|

Statement 1: f(x) is differentiable for all x > 0

Statement 2: f(x) is increasing in (1, ∞)

Statement 3: f(x) is decreasing in (0, 1)

Options

A

Statement 1 and statement 3 are true

B

All statements are correct

C

Statement 2 and statement 3 are correct

D

Statement 1 and statement 2 are correct

Show Answer

Correct Answer :

Option A

Statement 1 and statement 3 are true

Statement 1 and statement 3 are true

Solution :

We are given the function:
f(x)=|lnx|-|x-1|
defined for x>0.

Let's analyze the behaviors of the logarithmic term |lnx| and the linear term |x-1| on different intervals.

Case 1: When x1
For x1, we have lnx0 and x-10.
Thus, the absolute values can be opened directly:
f(x)=lnx-(x-1)=lnx-x+1

Case 2: When 0<x<1
For 0<x<1, we have lnx<0 and x-1<0.
Thus, the absolute values open with negative signs:
f(x)=-lnx-(-(x-1))=-lnx+x-1

Now we evaluate each statement step-by-step.

Evaluating Statement 1: Differentiability at x=1
Since f(x) is composed of differentiable functions in the intervals (0,1) and (1,), we only need to check differentiability at the boundary point x=1.
Let us find the left-hand derivative (LHD) and right-hand derivative (RHD) at x=1.

For 0<x<1:
f'(x)=ddx(-lnx+x-1)=-1x+1
Taking the limit as x1-:
LHD=-11+1=0

For x>1:
f'(x)=ddx(lnx-x+1)=1x-1
Taking the limit as x1+:
RHD=11-1=0

Since the Left-Hand Derivative equals the Right-Hand Derivative (LHD=RHD=0) at x=1, the function f(x) is differentiable at x=1. Since it is differentiable everywhere else in x>0, Statement 1 is true.

Evaluating Statement 2: Monotonicity in (1,)
For x>1, we have:
f'(x)=1x-1
Since x>1, we have 1x<1, which implies:
f'(x)<0
Therefore, f(x) is decreasing in (1,). Thus, Statement 2 is false.

Evaluating Statement 3: Monotonicity in (0,1)
For 0<x<1, we have:
f'(x)=-1x+1
Since 0<x<1, we have 1x>1, which implies:
-1x<-1-1x+1<0
Thus, f'(x)<0 for all x(0,1).
Therefore, f(x) is decreasing in (0,1). Thus, Statement 3 is true.

Consequently, Statement 1 and Statement 3 are true, while Statement 2 is false.

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