Question

Find the interval in which the function f(x) = x2 - 4x is strictly increasing ?
1. [2, ∞)
2. (2, ∞)
3. (0, ∞)
4. (-∞ , 2)
5. None of these

Answer 1

Correct Answer - Option 2 : (2, ∞)

Concept:  

If f′(x) > 0 at each point in an interval, then the function is said to be strictly increasing.

Calculations:

Given , f(x) = x2 - 4x 

Differentiating, we get

f'(x) = 2x - 4

f(x) is strictly increasing function

∴ f'(x) > 0

⇒ 2x - 4 > 0

⇒ x > 2

∴ x ∈ (2, ∞)