\int_{\gamma} \frac{xdy - ydx}{x^2 + y^2}= \int_{\alpha_{1}}^{\alpha_{2}} \frac {r^2cos^2t + r^2 sin^2t}{r^2cos^2t + r^2 sin^2t}dt = \\
\int_{\alpha_{1}}^{\alpha_{2}} dt = t |_{\alpha_{1}}^{\alpha_{2}}= \alpha_{2} - \alpha_{1}