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Application of Derivatives question

2017 · Shift 1 · Q36
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Application of Derivatives question

2017 · Shift 1 · Q36

JEE AdvancedMathematicsApplication of DerivativesMCQ+3 / −1
By approximately matching the information given in the three columns of the following table.

Let f(x) = x + loge x −-− x loge x, x ∈\in∈(0, ∞\infty∞)

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 x∈(1,e2)x \in (1,{e^2})x∈(1,e2) (i) lim⁡x→∞ f(x)=0\mathop {\lim }\limits_{x \to \infty } \,f(x) = 0x→∞lim​f(x)=0 f is increasing in (0, 1)
(ii) f'(x) = 0 for some x∈(1,e)x \in (1,e)x∈(1,e) lim⁡x→∞ f(x)=−∞\mathop {\lim }\limits_{x \to \infty } \,f(x) = - \inftyx→∞lim​f(x)=−∞ f is decreasing in (e, e2{e^2}e2)
(iii) f'(x) = 0 for some x∈(0,1)x \in (0,1)x∈(0,1) lim⁡x→∞ f′(x)=−∞\mathop {\lim }\limits_{x \to \infty } \,f'(x) = - \inftyx→∞lim​f′(x)=−∞ f' is increasing in (0, 1)
(iv) f'(x) = 0 for some x∈(1,e)x \in (1,e)x∈(1,e) lim⁡x→∞ f′(x)=0\mathop {\lim }\limits_{x \to \infty } \,f'(x) = 0x→∞lim​f′(x)=0 f' is decreasing in (e, e2{e^2}e2)
Which of the following options is the only CORRECT combination?
  1. A
    (III) (iii) (R)
  2. B
    (IV) (iv) (S)
  3. C
    (II) (ii) (Q)
  4. 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 f(x)=x+logex−xlogexf(x) = x + log_e x - x log_e xf(x)=x+loge​x−xloge​x, for x∈(0,∞)x \in (0, \infty)x∈(0,∞).

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: f(x)=x+ln⁡x−xln⁡xf(x) = x + \ln x - x \ln xf(x)=x+lnx−xlnx

First derivative: f′(x)=d/dx(x+ln⁡x−xln⁡x)f'(x) = d/dx (x + \ln x - x \ln x)f′(x)=d/dx(x+lnx−xlnx) f′(x)=1+1/x−(1⋅ln⁡x+x⋅1/x)f'(x) = 1 + 1/x - (1 \cdot \ln x + x \cdot 1/x)f′(x)=1+1/x−(1⋅lnx+x⋅1/x) f′(x)=1+1/x−ln⁡x−1f'(x) = 1 + 1/x - \ln x - 1f′(x)=1+1/x−lnx−1 f′(x)=1/x−ln⁡xf'(x) = 1/x - \ln xf′(x)=1/x−lnx

Second derivative: f′′(x)=d/dx(1/x−ln⁡x)f''(x) = d/dx (1/x - \ln x)f′′(x)=d/dx(1/x−lnx) f′′(x)=−1/x2−1/x=−(1/x2+1/x)f''(x) = -1/x^2 - 1/x = -(1/x^2 + 1/x)f′′(x)=−1/x2−1/x=−(1/x2+1/x) For the domain x∈(0,∞)x \in (0, \infty)x∈(0,∞), x > 0, so x2>0x^2 > 0x2>0. This implies 1/x > 0 and 1/x2>01/x^2 > 01/x2>0. Therefore, f''(x) is always negative for x∈(0,∞)x \in (0, \infty)x∈(0,∞).

Step 2: Evaluate statements in Column 1

(I) f(x) = 0 for some x∈(1,e2)x \in (1, e^2)x∈(1,e2)

  • f(1)=1+ln⁡(1)−1⋅ln⁡(1)=1+0−0=1>0f(1) = 1 + \ln(1) - 1 \cdot \ln(1) = 1 + 0 - 0 = 1 > 0f(1)=1+ln(1)−1⋅ln(1)=1+0−0=1>0
  • f(e2)=e2+ln⁡(e2)−e2ln⁡(e2)=e2+2−e2⋅2=2−e2f(e^2) = e^2 + \ln(e^2) - e^2 \ln(e^2) = e^2 + 2 - e^2 \cdot 2 = 2 - e^2f(e2)=e2+ln(e2)−e2ln(e2)=e2+2−e2⋅2=2−e2
  • Since e≈2.718e \approx 2.718e≈2.718, e2≈7.389e^2 \approx 7.389e2≈7.389, so f(e2)=2−e2<0f(e^2) = 2 - e^2 < 0f(e2)=2−e2<0.
  • As f(x) is continuous on [1,e2][1, e^2][1,e2] and f(1) > 0 and f(e2)<0f(e^2) < 0f(e2)<0, by the Intermediate Value Theorem, there exists a root in (1,e2)(1, e^2)(1,e2). Statement (I) is TRUE.

(II) f'(x) = 0 for some x∈(1,e)x \in (1, e)x∈(1,e)

  • f′(x)=1/x−ln⁡xf'(x) = 1/x - \ln xf′(x)=1/x−lnx
  • f′(1)=1/1−ln⁡(1)=1−0=1>0f'(1) = 1/1 - \ln(1) = 1 - 0 = 1 > 0f′(1)=1/1−ln(1)=1−0=1>0
  • f′(e)=1/e−ln⁡(e)=1/e−1<0f'(e) = 1/e - \ln(e) = 1/e - 1 < 0f′(e)=1/e−ln(e)=1/e−1<0
  • As f'(x) is continuous on [1, e] and f'(1) > 0 and f'(e) < 0, by the Intermediate Value Theorem, there exists a root in (1, e). Statement (II) is TRUE.

(III) f'(x) = 0 for some x∈(0,1)x \in (0, 1)x∈(0,1)

  • For x∈(0,1)x \in (0, 1)x∈(0,1), we have 1/x > 1 and ln⁡x<0\ln x < 0lnx<0.
  • Therefore, f′(x)=1/x−ln⁡x>1−0=1f'(x) = 1/x - \ln x > 1 - 0 = 1f′(x)=1/x−lnx>1−0=1. f'(x) is always positive on (0, 1). Statement (III) is FALSE.

(IV) f''(x) = 0 for some x∈(1,e)x \in (1, e)x∈(1,e)

  • We found f′′(x)=−(1/x2+1/x)f''(x) = -(1/x^2 + 1/x)f′′(x)=−(1/x2+1/x). For x > 0, f''(x) is always negative and never zero. Statement (IV) is FALSE.

Step 3: Evaluate statements in Column 2

(i) lim⁡x→∞f(x)=0\lim_{x \to \infty} f(x) = 0limx→∞​f(x)=0

  • f(x)=x+ln⁡x−xln⁡x=x(1−ln⁡x)+ln⁡xf(x) = x + \ln x - x \ln x = x(1 - \ln x) + \ln xf(x)=x+lnx−xlnx=x(1−lnx)+lnx
  • As x→∞x \to \inftyx→∞, (1−ln⁡x)→−∞(1 - \ln x) \to -\infty(1−lnx)→−∞. The term x(1−ln⁡x)x(1 - \ln x)x(1−lnx) dominates and goes to −∞-\infty−∞.
  • So, lim⁡x→∞f(x)=−∞\lim_{x \to \infty} f(x) = -\inftylimx→∞​f(x)=−∞. Statement (i) is FALSE.

(ii) lim⁡x→∞f(x)=−∞\lim_{x \to \infty} f(x) = -\inftylimx→∞​f(x)=−∞

  • As shown above, this statement is TRUE.

(iii) lim⁡x→∞f′(x)=−∞\lim_{x \to \infty} f'(x) = -\inftylimx→∞​f′(x)=−∞

  • f′(x)=1/x−ln⁡xf'(x) = 1/x - \ln xf′(x)=1/x−lnx
  • As x→∞x \to \inftyx→∞, 1/x→01/x \to 01/x→0 and ln⁡x→∞\ln x \to \inftylnx→∞.
  • lim⁡x→∞f′(x)=0−∞=−∞\lim_{x \to \infty} f'(x) = 0 - \infty = -\inftylimx→∞​f′(x)=0−∞=−∞. Statement (iii) is TRUE.

(iv) lim⁡x→∞f′(x)=0\lim_{x \to \infty} f'(x) = 0limx→∞​f′(x)=0

  • The limit is −∞-\infty−∞, 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) > 0 for x∈(0,1)x \in (0, 1)x∈(0,1). Thus, f is increasing on (0, 1). Statement (P) is TRUE.

(Q) f is decreasing in (e,e2)(e, e^2)(e,e2)

  • We check the sign of f'(x) on (e,e2)(e, e^2)(e,e2). Let x0x_0x0​ be the root of f'(x) = 0, where x0∈(1,e)x_0 \in (1, e)x0​∈(1,e).
  • Since f''(x) < 0, f'(x) is a strictly decreasing function. For x>x0x > x_0x>x0​, f′(x)<f′(x0)=0f'(x) < f'(x_0) = 0f′(x)<f′(x0​)=0.
  • The interval (e,e2)(e, e^2)(e,e2) is in the region x>x0x > x_0x>x0​ (since e>x0e > x_0e>x0​). So, f'(x) < 0 on (e,e2)(e, e^2)(e,e2).
  • Thus, f is decreasing in (e,e2)(e, e^2)(e,e2). Statement (Q) is TRUE.

(R) f' is increasing in (0, 1)

  • The rate of change of f' is given by f''(x). We found f''(x) < 0 for all x > 0. This means f' is a strictly decreasing function on its entire domain, including (0, 1). Statement (R) is FALSE.

(S) f' is decreasing in (e,e2)(e, e^2)(e,e2)

  • Since f''(x) < 0 for all x > 0, f' is decreasing on (e,e2)(e, e^2)(e,e2). 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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