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The value of the integral int(0)^(pi)(e^...

The value of the integral `int_(0)^(pi)(e^(|cosx|)sinx)/(1+e^(cotx))dx` is equal to

A

`e+1`

B

`1-e`

C

`e-1`

D

`(-1+e)/(2)`

Text Solution

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The correct Answer is:
To solve the integral \[ I = \int_{0}^{\pi} \frac{e^{|\cos x|} \sin x}{1 + e^{\cot x}} \, dx, \] we can use the property of definite integrals which states that \[ \int_{a}^{b} f(x) \, dx = \int_{a}^{b} f(a + b - x) \, dx. \] In our case, \( a = 0 \) and \( b = \pi \), so we have: \[ I = \int_{0}^{\pi} f(\pi - x) \, dx. \] First, we need to compute \( f(\pi - x) \): 1. **Calculate \( f(\pi - x) \)**: \[ f(\pi - x) = \frac{e^{|\cos(\pi - x)|} \sin(\pi - x)}{1 + e^{\cot(\pi - x)}} \] Since \( \cos(\pi - x) = -\cos x \), we have \( |\cos(\pi - x)| = |\cos x| \) and \( \sin(\pi - x) = \sin x \). Also, \( \cot(\pi - x) = -\cot x \). Therefore, \[ f(\pi - x) = \frac{e^{|\cos x|} \sin x}{1 + e^{-\cot x}}. \] 2. **Rewrite \( f(\pi - x) \)**: \[ f(\pi - x) = \frac{e^{|\cos x|} \sin x}{1 + \frac{1}{e^{\cot x}}} = \frac{e^{|\cos x|} \sin x \cdot e^{\cot x}}{e^{\cot x} + 1}. \] 3. **Add \( I \) and \( f(\pi - x) \)**: Now, we can add \( I \) and \( f(\pi - x) \): \[ 2I = \int_{0}^{\pi} \left( f(x) + f(\pi - x) \right) \, dx. \] This gives: \[ 2I = \int_{0}^{\pi} \left( \frac{e^{|\cos x|} \sin x}{1 + e^{\cot x}} + \frac{e^{|\cos x|} \sin x \cdot e^{\cot x}}{e^{\cot x} + 1} \right) \, dx. \] 4. **Simplify the expression**: The denominators are the same, so we can combine the fractions: \[ 2I = \int_{0}^{\pi} \frac{e^{|\cos x|} \sin x (1 + e^{\cot x})}{1 + e^{\cot x}} \, dx. \] This simplifies to: \[ 2I = \int_{0}^{\pi} e^{|\cos x|} \sin x \, dx. \] 5. **Evaluate the integral**: Now we can evaluate: \[ I = \frac{1}{2} \int_{0}^{\pi} e^{|\cos x|} \sin x \, dx. \] 6. **Change of variable**: Let \( t = \cos x \), then \( dt = -\sin x \, dx \). The limits change from \( x = 0 \) (where \( t = 1 \)) to \( x = \pi \) (where \( t = -1 \)): \[ I = \frac{1}{2} \int_{1}^{-1} e^{|t|} (-dt) = \frac{1}{2} \int_{-1}^{1} e^{|t|} \, dt. \] 7. **Split the integral**: We can split this integral into two parts: \[ I = \frac{1}{2} \left( \int_{-1}^{0} e^{-t} \, dt + \int_{0}^{1} e^{t} \, dt \right). \] 8. **Evaluate the integrals**: - For \( \int_{-1}^{0} e^{-t} \, dt \): \[ = \left[ -e^{-t} \right]_{-1}^{0} = -e^{0} + e^{1} = 1 - e. \] - For \( \int_{0}^{1} e^{t} \, dt \): \[ = \left[ e^{t} \right]_{0}^{1} = e - 1. \] 9. **Combine results**: Thus, \[ I = \frac{1}{2} \left( (1 - e) + (e - 1) \right) = \frac{1}{2} \cdot 2(e - 1) = e - 1. \] Therefore, the value of the integral is \[ \boxed{e - 1}. \]
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