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Differentiation question

2008 · Shift 2 · Q35
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  5. /2008 · Shift 2 · Q35

Differentiation question

2008 · Shift 2 · Q35

JEE AdvancedMathematicsDifferentiationMCQ+3 / −1
Let g(x)=log⁡f(x)g(x) = \log f(x)g(x)=logf(x), where f(x)f(x)f(x) is a twice differentiable positive function on (0, ∞\infty∞) such that f(x+1)=xf(x)f(x + 1) = xf(x)f(x+1)=xf(x). Then for N = 1, 2, 3, ..., g′′(N+12)−g′′(12)=g''\left( {N + {1 \over 2}} \right) - g''\left( {{1 \over 2}} \right) =g′′(N+21​)−g′′(21​)=
  1. A
    −4{1+19+125+.......+1(2N−1)2}- 4\left\{ {1 + {1 \over 9} + {1 \over {25}} + ....... + {1 \over {{{\left( {2N - 1} \right)}^2}}}} \right\}−4{1+91​+251​+.......+(2N−1)21​}
  2. B
    4{1+19+125+.......+1(2N−1)2}4\left\{ {1 + {1 \over 9} + {1 \over {25}} + ....... + {1 \over {{{\left( {2N - 1} \right)}^2}}}} \right\}4{1+91​+251​+.......+(2N−1)21​}
  3. C
    −4{1+19+125+.......+1(2N+1)2}- 4\left\{ {1 + {1 \over 9} + {1 \over {25}} + ....... + {1 \over {{{\left( {2N + 1} \right)}^2}}}} \right\}−4{1+91​+251​+.......+(2N+1)21​}
  4. D
    4{1+19+125+.......+1(2N+1)2}4\left\{ {1 + {1 \over 9} + {1 \over {25}} + ....... + {1 \over {{{\left( {2N + 1} \right)}^2}}}} \right\}4{1+91​+251​+.......+(2N+1)21​}
View written solutionFree

Correct answer: A

Step-by-step Solution:

  1. Analyze the given functions and relations. We are given g(x)=log⁡f(x)g(x) = \log f(x)g(x)=logf(x), where f(x) is a twice differentiable positive function on (0,∞)(0, \infty)(0,∞). The function f(x) satisfies the functional equation f(x + 1) = xf(x).

  2. Establish a relationship for g(x) using the functional equation. Take the natural logarithm of both sides of the functional equation for f(x): log⁡(f(x+1))=log⁡(xf(x))\log(f(x + 1)) = \log(xf(x))log(f(x+1))=log(xf(x)) Using the properties of logarithms, we can write: log⁡(f(x+1))=log⁡(x)+log⁡(f(x))\log(f(x + 1)) = \log(x) + \log(f(x))log(f(x+1))=log(x)+log(f(x)) From the definition g(x)=log⁡f(x)g(x) = \log f(x)g(x)=logf(x), we have g(x+1)=log⁡f(x+1)g(x + 1) = \log f(x+1)g(x+1)=logf(x+1). Substituting this into the equation above gives: g(x+1)=log⁡(x)+g(x)g(x + 1) = \log(x) + g(x)g(x+1)=log(x)+g(x)

  3. Differentiate the relation for g(x) to find a relation for its derivatives. Differentiate the equation g(x+1)=log⁡(x)+g(x)g(x + 1) = \log(x) + g(x)g(x+1)=log(x)+g(x) with respect to x: ddxg(x+1)=ddx(log⁡(x)+g(x)){d \over dx} g(x+1) = {d \over dx} (\log(x) + g(x))dxd​g(x+1)=dxd​(log(x)+g(x)) Applying the chain rule, we get: g′(x+1)⋅1=1x+g′(x)g'(x + 1) \cdot 1 = {1 \over x} + g'(x)g′(x+1)⋅1=x1​+g′(x) g′(x+1)=1x+g′(x)g'(x + 1) = {1 \over x} + g'(x)g′(x+1)=x1​+g′(x) Now, differentiate this equation again with respect to x: ddxg′(x+1)=ddx(1x+g′(x)){d \over dx} g'(x+1) = {d \over dx} \left({1 \over x} + g'(x)\right)dxd​g′(x+1)=dxd​(x1​+g′(x)) g′′(x+1)⋅1=−1x2+g′′(x)g''(x + 1) \cdot 1 = -{1 \over x^2} + g''(x)g′′(x+1)⋅1=−x21​+g′′(x) This gives us a recurrence relation for g''(x): g′′(x+1)−g′′(x)=−1x2g''(x + 1) - g''(x) = -{1 \over x^2}g′′(x+1)−g′′(x)=−x21​

  4. Evaluate the expression g''(N + 1/2) - g''(1/2) using the recurrence relation. The expression can be written as a telescoping sum: g′′(N+12)−g′′(12)=∑k=1N[g′′(k+12)−g′′(k−12)]g''\left(N + {1 \over 2}\right) - g''\left({1 \over 2}\right) = \sum_{k=1}^{N} \left[ g''\left(k + {1 \over 2}\right) - g''\left(k - {1 \over 2}\right) \right]g′′(N+21​)−g′′(21​)=∑k=1N​[g′′(k+21​)−g′′(k−21​)] Let's verify this sum: For k=1: g''(3/2) - g''(1/2) For k=2: g''(5/2) - g''(3/2) ... For k=N: g''(N+1/2) - g''(N-1/2) Summing these terms, all intermediate terms cancel out, leaving g''(N+1/2) - g''(1/2).

  5. Apply the recurrence relation to the terms in the sum. Our recurrence relation is g′′(x+1)−g′′(x)=−1/x2g''(x + 1) - g''(x) = -1/x^2g′′(x+1)−g′′(x)=−1/x2. For each term in the sum, let x = k - 1/2. Then x + 1 = k - 1/2 + 1 = k + 1/2. Substituting x = k - 1/2 into the recurrence relation gives: g′′(k+12)−g′′(k−12)=−1(k−1/2)2=−1(2k−12)2=−4(2k−1)2g''\left(k + {1 \over 2}\right) - g''\left(k - {1 \over 2}\right) = -{1 \over (k - 1/2)^2} = -{1 \over (\frac{2k-1}{2})^2} = -{4 \over (2k-1)^2}g′′(k+21​)−g′′(k−21​)=−(k−1/2)21​=−(22k−1​)21​=−(2k−1)24​

  6. Calculate the final sum. Now we can substitute this back into our telescoping sum expression: g′′(N+12)−g′′(12)=∑k=1N(−4(2k−1)2)g''\left(N + {1 \over 2}\right) - g''\left({1 \over 2}\right) = \sum_{k=1}^{N} \left( -{4 \over (2k-1)^2} \right)g′′(N+21​)−g′′(21​)=∑k=1N​(−(2k−1)24​) g′′(N+12)−g′′(12)=−4∑k=1N1(2k−1)2g''\left(N + {1 \over 2}\right) - g''\left({1 \over 2}\right) = -4 \sum_{k=1}^{N} {1 \over (2k-1)^2}g′′(N+21​)−g′′(21​)=−4∑k=1N​(2k−1)21​ Expanding the sum: −4[1(2(1)−1)2+1(2(2)−1)2+1(2(3)−1)2+⋯+1(2N−1)2]-4 \left[ {1 \over (2(1)-1)^2} + {1 \over (2(2)-1)^2} + {1 \over (2(3)-1)^2} + \dots + {1 \over (2N-1)^2} \right]−4[(2(1)−1)21​+(2(2)−1)21​+(2(3)−1)21​+⋯+(2N−1)21​] −4[112+132+152+⋯+1(2N−1)2]-4 \left[ {1 \over 1^2} + {1 \over 3^2} + {1 \over 5^2} + \dots + {1 \over (2N-1)^2} \right]−4[121​+321​+521​+⋯+(2N−1)21​] −4{1+19+125+⋯+1(2N−1)2}-4 \left\{ 1 + {1 \over 9} + {1 \over 25} + \dots + {1 \over (2N-1)^2} \right\}−4{1+91​+251​+⋯+(2N−1)21​}

  7. Compare with the given options. The result matches option A.

Final Answer is option A.

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