Answer
$-2e^{-\frac{x}{2}}+C$
Work Step by Step
Let $u=-\frac{x}{2}$. Then $du=-\frac{1}{2}dx$, so $dx=-2du$.
$\int e^{-x/2}dx$
$=\int e^u*(-2)du$
$=-2e^u+C$
$=-2e^{-\frac{x}{2}}+C$
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