MCQ
If the integral $\int_{0}^{10} \frac{[\sin 2 \pi x ]}{ e ^{ x -[ x ]}} dx =\alpha e ^{-1}+\beta e ^{-\frac{1}{2}}+\gamma$, where $\alpha, \beta, \gamma$ are integers and $[ x ]$ denotes the greatest integer less than or equal to $x$, then the value of $\alpha+\beta+\gamma$ is equal to ........ .
  • $0$
  • B
    $20$
  • C
    $25$
  • D
    $10$

Answer

Correct option: A.
$0$
a
Let $I=\int_{0}^{10} \frac{[\sin 2 \pi x ]}{ e ^{ x -[ x ]}} dx =\int_{0}^{10} \frac{[\sin 2 \pi x ]}{ e ^{\{ x \}}} dx$

Function $f ( x )=\frac{[\sin 2 \pi x ]}{ e ^{\{ x \}}}$ is periodic with

period $'1'$ Therefore

$I=10 \int_{0}^{1} \frac{[\sin 2 \pi x ]}{ e ^{\{ x \}}} d x$

$=10 \int_{0}^{1} \frac{[\sin 2 \pi x ]}{ e ^{ x }} d x$

$=10\left(\int_{0}^{1 / 2} \frac{[\sin 2 \pi x ]}{ e ^{ x }} d x +\int_{1 / 2}^{1} \frac{[\sin 2 \pi x ]}{ e ^{ x }} dx \right)$

$=10\left(0+\int_{1 / 2}^{1} \frac{(-1)}{ e ^{ x }} dx \right)$

$=-10 \int_{1 / 2}^{1} e ^{- x } dx$

$=10\left( e ^{-1}- e ^{-1 / 2}\right)$

Now,

$10 \cdot e ^{-1}-10 \cdot e ^{-1 / 2}=\alpha e ^{-1}+\beta e ^{-1 / 2}+\gamma($ given $)$

$\Rightarrow \alpha=10, \beta=-10, \gamma=0$

$\Rightarrow \alpha+\beta+\gamma=0$

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