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Question

Which of the following function are monotonic in the interval (0,1) ?


A

f(x)=x2

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B

f(x)=1x

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C

f(x)=x3x

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D

f(x)=ln(x)

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Solution

The correct option is D

f(x)=ln(x)


To figure out the intervals in which a function is monotonous we’ll differentiate the function and find the intervals in which the derivative has the same sign

Function will be monotonic when f’(x) > 0 or f’(x) < 0 or f’(x) 0 or f’(x) 0.

Let’s differentiate each option one by one.

  1. f(x) = x2

In the interval (0,1) f’(x) > 0

So we can say f(x) = x2 is monotonic in (0,1)

B. f(x) = 1/x

f’(x) = -1 / x2

Here, we can see that f’(x) 0 in the interval (0,1)

So the function is monotonically increasing.

C. f(x) = x3- x

f’(x) = 3 x2-1

f’(x) will be positive in the intervals of -

3 x2-1 ≥ 0

x2- 1/3 ≥ 0

xϵ(,1/3]U[1/3,)ϵ

And x213 will be negative in the interval [- 1 / 3, 1 / 3]

In the interval (0,1)for one part of the interval that is (0, 1 / 3 ] function will be decreasing and in [1 / 3 , 1) it’ll be increasing.

Hence, the function is not monotonic in the interval (0,1).

D. f(x) = ln(x)

f’(x) = 1/x

f’(x) is positive in the interval of (0,1) and thus monotonic.


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