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NDA 2025
Exam
NDA
Year
2025
Subject
Mathematics
Chapter
Application of Derivatives

NDA 2025 · Mathematics · Application of Derivatives

Consider the following for the two (02) items that follow: Let the...

Consider the following for the two (02) items that follow:
Let the function f(x)=x2+9f(x) = x^2 + 9.

Consider the following statements:
I. f(x)f(x) is an increasing function.
II. f(x)f(x) has local maximum at x=0x = 0.
Which of the statements given above is/are correct?

Options

  1. AI only
  2. BII only
  3. CBoth I and II
  4. DNeither I nor II

Correct answer

D. Neither I nor II

Step-by-step solution

Given f(x)=x2+9f(x) = x^2 + 9.

Differentiating with respect to xx, we get f′(x)=2xf'(x) = 2x.

For x<0x < 0, f′(x)<0f'(x) < 0, which means f(x)f(x) is strictly decreasing in (−∞,0)(-\infty, 0).

For x>0x > 0, f′(x)>0f'(x) > 0, which means f(x)f(x) is strictly increasing in (0,∞)(0, \infty).

Since f(x)f(x) is not increasing on its entire domain, Statement I is incorrect.

To find local extrema, we set f′(x)=0f'(x) = 0, which gives x=0x = 0.

The second derivative is f′′(x)=2f''(x) = 2.

Since f′′(0)=2>0f''(0) = 2 > 0, f(x)f(x) has a local minimum at x=0x = 0, not a local maximum.

Thus, Statement II is also incorrect.

Both statements are incorrect.

Answer: Neither I nor II
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