89. $\int \sin \frac{x}{2} d x$.

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89. $\int \sin \frac{x}{2} d x$.
Solution. $\int \sin \frac{x}{2} d x=2 \int \sin \frac{x}{2} d\left(\frac{x}{2}\right)=-2 \cos \frac{x}{2}+C$.