Q243 / 269 · NDA 2025 (Phase 1) · Mathematics
Consider the following for the two (02) items that follow:
Let the function $f(x) = x^2 + 9$.
Consider the following statements:
I. $f(x)$ is an increasing function.
II. $f(x)$ has local maximum at $x = 0$.
Which of the statements given above is/are correct?
Let the function $f(x) = x^2 + 9$.
Consider the following statements:
I. $f(x)$ is an increasing function.
II. $f(x)$ has local maximum at $x = 0$.
Which of the statements given above is/are correct?
- AI only
- BII only
- CBoth I and II
- Correct answerDNeither I nor II
Solution
Given $f(x) = x^2 + 9$.
Differentiating with respect to $x$, we get $f'(x) = 2x$.
For $x < 0$, $f'(x) < 0$, which means $f(x)$ is strictly decreasing in $(-\infty, 0)$.
For $x > 0$, $f'(x) > 0$, which means $f(x)$ is strictly increasing in $(0, \infty)$.
Since $f(x)$ is not increasing on its entire domain, Statement I is incorrect.
To find local extrema, we set $f'(x) = 0$, which gives $x = 0$.
The second derivative is $f''(x) = 2$.
Since $f''(0) = 2 > 0$, $f(x)$ has a local minimum at $x = 0$, not a local maximum.
Thus, Statement II is also incorrect.
Both statements are incorrect.
Answer: Neither I nor II
Differentiating with respect to $x$, we get $f'(x) = 2x$.
For $x < 0$, $f'(x) < 0$, which means $f(x)$ is strictly decreasing in $(-\infty, 0)$.
For $x > 0$, $f'(x) > 0$, which means $f(x)$ is strictly increasing in $(0, \infty)$.
Since $f(x)$ is not increasing on its entire domain, Statement I is incorrect.
To find local extrema, we set $f'(x) = 0$, which gives $x = 0$.
The second derivative is $f''(x) = 2$.
Since $f''(0) = 2 > 0$, $f(x)$ has a local minimum at $x = 0$, not a local maximum.
Thus, Statement II is also incorrect.
Both statements are incorrect.
Answer: Neither I nor II
Hence, the correct answer is option: 'D'.