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\[\setLTR x_2ox_1^{-1} =
\begin{cases}
\theta & \text{ $,\  \ 0<\theta<\pi$} \\
0 & ,\  \  \text{\theta =\pi$}  \\
\theta-2\pi &,\  \ \text{\pi< \theta <2\pi }  
\end{cases} \]




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