Which of the following function are monotonic in the interval (0,1) ?
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.
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 x2−13 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.