Let f(x) = x + loge x x loge x, x (0, )
Column 1 contains information about zeroes of f(x), f'(x) and f"(x).
Column 2 contains information about the limiting behaviour of f(x), f'(x) and f"(x) at infinity.
Column 3 contains information about increasing/decreasing nature of f(x) and f'(x).
| Column - 1 | Column - 2 | Column - 3 | |
|---|---|---|---|
| (i) | f(x) = 0 for some | (i) | f is increasing in (0, 1) |
| (ii) | f'(x) = 0 for some | f is decreasing in (e, ) | |
| (iii) | f'(x) = 0 for some | f' is increasing in (0, 1) | |
| (iv) | f'(x) = 0 for some | f' is decreasing in (e, ) |
- A(III) (iii) (R)
- B(IV) (iv) (S)
- C(II) (ii) (Q)
- D(I0 (i) (P)
View written solutionFree
Correct answer: C
The user wants me to solve a matching-style question from JEE Mathematics, specifically from the topic of Application of Derivatives.
The function given is , for .
The question asks to find the correct combination of statements from three columns.
Let's analyze the function f(x) and its derivatives f'(x) and f''(x) step-by-step.
Step 1: Analyze the function f(x) and its derivatives
Given function:
First derivative:
Second derivative:
For the domain , x > 0, so . This implies 1/x > 0 and .
Therefore, f''(x) is always negative for .
Step 2: Evaluate statements in Column 1
(I) f(x) = 0 for some
- Since , , so .
- As
f(x)is continuous on andf(1) > 0and , by the Intermediate Value Theorem, there exists a root in . Statement (I) is TRUE.
(II) f'(x) = 0 for some
- As
f'(x)is continuous on[1, e]andf'(1) > 0andf'(e) < 0, by the Intermediate Value Theorem, there exists a root in(1, e). Statement (II) is TRUE.
(III) f'(x) = 0 for some
- For , we have
1/x > 1and . - Therefore, .
f'(x)is always positive on(0, 1). Statement (III) is FALSE.
(IV) f''(x) = 0 for some
- We found . For
x > 0,f''(x)is always negative and never zero. Statement (IV) is FALSE.
Step 3: Evaluate statements in Column 2
(i)
- As , . The term dominates and goes to .
- So, . Statement (i) is FALSE.
(ii)
- As shown above, this statement is TRUE.
(iii)
- As , and .
- . Statement (iii) is TRUE.
(iv)
- The limit is , not 0. Statement (iv) is FALSE.
Step 4: Evaluate statements in Column 3
(P) f is increasing in (0, 1)
- We check the sign of
f'(x)on(0, 1). As shown in the analysis of (III),f'(x) > 0for . Thus,fis increasing on(0, 1). Statement (P) is TRUE.
(Q) f is decreasing in
- We check the sign of
f'(x)on . Let be the root off'(x) = 0, where . - Since
f''(x) < 0,f'(x)is a strictly decreasing function. For , . - The interval is in the region (since ). So,
f'(x) < 0on . - Thus,
fis decreasing in . Statement (Q) is TRUE.
(R) f' is increasing in (0, 1)
- The rate of change of
f'is given byf''(x). We foundf''(x) < 0for allx > 0. This meansf'is a strictly decreasing function on its entire domain, including(0, 1). Statement (R) is FALSE.
(S) f' is decreasing in
- Since
f''(x) < 0for allx > 0,f'is decreasing on . Statement (S) is TRUE.
Step 5: Check the combinations
- A: (III) (iii) (R) -> (III) is False. Incorrect.
- B: (IV) (iv) (S) -> (IV) is False. Incorrect.
- C: (II) (ii) (Q) -> (II) is True, (ii) is True, (Q) is True. This combination is correct.
- D: (I) (i) (P) -> (i) is False. Incorrect.
The only combination where all three statements are correct is (II), (ii), (Q).
Therefore, option C is the correct answer.
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