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Suppose f, f' and f'' are continuous on ...

Suppose f, f' and f'' are continuous on [0, e] and that `f'(e )= f(e ) = f(1) = 1` and `int_(1)^(e )(f(x))/(x^(2))dx=(1)/(2)`, then the value of `int_(1)^(e ) f''(x)ln xdx` equals :

A

0

B

1

C

2

D

None of these

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The correct Answer is:
To solve the problem, we need to evaluate the integral \( I = \int_{1}^{e} f''(x) \ln x \, dx \) given the conditions \( f'(e) = f(e) = f(1) = 1 \) and \( \int_{1}^{e} \frac{f(x)}{x^2} \, dx = \frac{1}{2} \). ### Step-by-Step Solution: 1. **Integration by Parts**: We will use integration by parts on the integral \( I = \int_{1}^{e} f''(x) \ln x \, dx \). We set: - \( u = \ln x \) (which gives \( du = \frac{1}{x} dx \)) - \( dv = f''(x) dx \) (which gives \( v = f'(x) \)) Using the integration by parts formula \( \int u \, dv = uv - \int v \, du \), we have: \[ I = \left[ \ln x \cdot f'(x) \right]_{1}^{e} - \int_{1}^{e} f'(x) \cdot \frac{1}{x} \, dx \] 2. **Evaluate the Boundary Terms**: Now we evaluate the boundary terms: - At \( x = e \): \( \ln e = 1 \) and \( f'(e) = 1 \), so \( \ln e \cdot f'(e) = 1 \cdot 1 = 1 \). - At \( x = 1 \): \( \ln 1 = 0 \) and \( f'(1) \) is unknown, but it will be multiplied by 0, so this term contributes 0. Thus, the boundary term evaluates to: \[ \left[ \ln x \cdot f'(x) \right]_{1}^{e} = 1 - 0 = 1 \] 3. **Substituting Back into the Integral**: Now we substitute back into the integral: \[ I = 1 - \int_{1}^{e} \frac{f'(x)}{x} \, dx \] 4. **Evaluate the Remaining Integral**: We need to evaluate \( \int_{1}^{e} \frac{f'(x)}{x} \, dx \). We can use the substitution \( f(x) = x^2 g(x) \) (where \( g(x) = \frac{f(x)}{x^2} \)). The given condition states: \[ \int_{1}^{e} \frac{f(x)}{x^2} \, dx = \frac{1}{2} \] This implies: \[ \int_{1}^{e} g(x) \, dx = \frac{1}{2} \] 5. **Relate \( f'(x) \) to \( g(x) \)**: We know that: \[ f'(x) = 2x g(x) + x^2 g'(x) \] Thus, \[ \int_{1}^{e} \frac{f'(x)}{x} \, dx = \int_{1}^{e} \left( 2g(x) + x g'(x) \right) \, dx \] 6. **Evaluate the Integral**: - The first part gives \( 2 \int_{1}^{e} g(x) \, dx = 2 \cdot \frac{1}{2} = 1 \). - The second part \( \int_{1}^{e} g'(x) \, dx = g(e) - g(1) \). Since \( g(1) = \frac{f(1)}{1^2} = 1 \) and \( g(e) = \frac{f(e)}{e^2} = \frac{1}{e^2} \), we have: \[ \int_{1}^{e} g'(x) \, dx = \frac{1}{e^2} - 1 \] 7. **Combine Results**: Therefore, \[ \int_{1}^{e} \frac{f'(x)}{x} \, dx = 1 + \left( \frac{1}{e^2} - 1 \right) = \frac{1}{e^2} \] 8. **Final Calculation**: Substitute this back into our expression for \( I \): \[ I = 1 - \frac{1}{e^2} \] ### Conclusion: The value of the integral \( \int_{1}^{e} f''(x) \ln x \, dx \) is: \[ \boxed{1 - \frac{1}{e^2}} \]
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