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Application of Derivatives question

2017 · Shift 1 · Q34
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  5. /2017 · Shift 1 · Q34

Application of Derivatives question

2017 · Shift 1 · Q34

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 INCORRECT combination?
  1. A
    (I) (iii) (P)
  2. B
    (II) (iv) (Q)
  3. C
    (II) (ii) (P)
  4. D
    (III) (i) (R)
View written solutionFree

Correct answer: D

  1. Given function

    f(x)=x+ln⁡x−xln⁡x,x∈(0,∞)f(x)=x+\ln x-x\ln x, \qquad x\in(0,\infty)f(x)=x+lnx−xlnx,x∈(0,∞)

  2. Find derivatives

    f′(x)=1+1x−(ln⁡x+1)=1x−ln⁡xf'(x)=1+\frac1x-(\ln x+1)=\frac1x-\ln xf′(x)=1+x1​−(lnx+1)=x1​−lnx

    f′′(x)=−1x2−1x=−1+xx2<0∀x>0f''(x)=-\frac1{x^2}-\frac1x=-\frac{1+x}{x^2}<0 \quad \forall x>0f′′(x)=−x21​−x1​=−x21+x​<0∀x>0


  1. Match Column 1

    We analyze the zeroes.

    (I) Zero of f(x)f(x)f(x)

    Solve approximately: x+ln⁡x−xln⁡x=0x+\ln x-x\ln x=0x+lnx−xlnx=0

    Check values: f(1)=1>0f(1)=1>0f(1)=1>0 f(e2)=e2+2−2e2=2−e2<0f(e^2)=e^2+2-2e^2=2-e^2<0f(e2)=e2+2−2e2=2−e2<0

    Since fff is continuous, there is some root in (1,e2)(1,e^2)(1,e2).

    So (I) is true.

    Zero of f′(x)f'(x)f′(x)

    f′(x)=1x−ln⁡xf'(x)=\frac1x-\ln xf′(x)=x1​−lnx

    Check: f′(1)=1>0,f′(e)=1e−1<0f'(1)=1>0, \qquad f'(e)=\frac1e-1<0f′(1)=1>0,f′(e)=e1​−1<0

    Hence by continuity, f′(x)=0f'(x)=0f′(x)=0 for some x∈(1,e)x\in(1,e)x∈(1,e).

    Also for x∈(0,1)x\in(0,1)x∈(0,1), ln⁡x<0,1x>0  ⟹  f′(x)=1x−ln⁡x>0\ln x<0, \quad \frac1x>0 \implies f'(x)=\frac1x-\ln x>0lnx<0,x1​>0⟹f′(x)=x1​−lnx>0 so there is no zero in (0,1)(0,1)(0,1).

    Therefore:

    • statement (ii): true
    • statement (iii): false
    • statement (iv) same as (ii): true

  1. Match Column 2

    Limit of f(x)f(x)f(x) as x→∞x\to\inftyx→∞

    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

    Since xln⁡xx\ln xxlnx dominates xxx and ln⁡x\ln xlnx, lim⁡x→∞f(x)=−∞\lim_{x\to\infty} f(x)=-\inftylimx→∞​f(x)=−∞

    So (ii) is correct, (i) is false.

    Limit of f′(x)f'(x)f′(x) as x→∞x\to\inftyx→∞

    f′(x)=1x−ln⁡x→0−∞=−∞f'(x)=\frac1x-\ln x \to 0-\infty=-\inftyf′(x)=x1​−lnx→0−∞=−∞

    So (iii) is correct, (iv) is false.


  1. Match Column 3

    Since f′(x)=1x−ln⁡xf'(x)=\frac1x-\ln xf′(x)=x1​−lnx

    • On (0,1)(0,1)(0,1), ln⁡x<0\ln x<0lnx<0, so f′(x)>0f'(x)>0f′(x)>0; hence fff is increasing on (0,1)(0,1)(0,1).
    • On (e,e2)(e,e^2)(e,e2), ln⁡x>1\ln x>1lnx>1 and 1/x<1/e<11/x<1/e<11/x<1/e<1, so f′(x)<0f'(x)<0f′(x)<0; hence fff is decreasing on (e,e2)(e,e^2)(e,e2).

    Also, f′′(x)=−1+xx2<0f''(x)=-\frac{1+x}{x^2}<0f′′(x)=−x21+x​<0 for all x>0x>0x>0, hence f′f'f′ is decreasing on every interval, including (e,e2)(e,e^2)(e,e2). Therefore:

    • P: fff increasing in (0,1)(0,1)(0,1) → true
    • Q: fff decreasing in (e,e2)(e,e^2)(e,e2) → true
    • R: f′f'f′ increasing in (0,1)(0,1)(0,1) → false
    • S: f′f'f′ decreasing in (e,e2)(e,e^2)(e,e2) → true

    From the options, the third-column labels used are:

    • Option A uses (P) = fff increasing in (0,1)(0,1)(0,1)
    • Option B uses (Q) = fff decreasing in (e,e2)(e,e^2)(e,e2)
    • Option C uses (P) = fff increasing in (0,1)(0,1)(0,1)
    • Option D uses (R) = f′f'f′ increasing in (0,1)(0,1)(0,1)

  1. Check each option

    Option A: (I),(iii),(P)(I),(iii),(P)(I),(iii),(P)

    • (I)(I)(I) : true
    • (iii)(iii)(iii) in Column 2 means lim⁡x→∞f′(x)=−∞\lim_{x\to\infty} f'(x)=-\inftylimx→∞​f′(x)=−∞ : true
    • (P)(P)(P) : true

    So A is correct combination.

    Option B: (II),(iv),(Q)(II),(iv),(Q)(II),(iv),(Q)

    • (II)(II)(II) : f′(x)=0f'(x)=0f′(x)=0 for some x∈(1,e)x\in(1,e)x∈(1,e) : true
    • (iv)(iv)(iv) in Column 2: lim⁡x→∞f′(x)=0\lim_{x\to\infty} f'(x)=0limx→∞​f′(x)=0 : false
    • (Q)(Q)(Q) : true

    So B is incorrect combination.

    Option C: (II),(ii),(P)(II),(ii),(P)(II),(ii),(P)

    • (II)(II)(II) : true
    • (ii)(ii)(ii) in Column 2: lim⁡x→∞f(x)=−∞\lim_{x\to\infty} f(x)=-\inftylimx→∞​f(x)=−∞ : true
    • (P)(P)(P) : true

    So C is correct combination.

    Option D: (III),(i),(R)(III),(i),(R)(III),(i),(R)

    • (III)(III)(III) : f′(x)=0f'(x)=0f′(x)=0 for some x∈(0,1)x\in(0,1)x∈(0,1) : false
    • (i)(i)(i) in Column 2: lim⁡x→∞f(x)=0\lim_{x\to\infty} f(x)=0limx→∞​f(x)=0 : false
    • (R)(R)(R) : false

    So D is also incorrect combination.


  1. Conclusion

    From direct analysis, both B and D are incorrect, so the statement "only incorrect combination" is inconsistent with the data as typed.

    However, among the given single-choice options, D is certainly incorrect, and this matches the stored answer.

    But strictly speaking, B is also incorrect.

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