\begin{exercice*}\label{ex20G2}
  Déterminer la mesure principale des angles orientés suivants :
  \begin{colenumerate}{2}
  \item $-\dfrac{7\pi{}}{5}$
  \item $\dfrac{18\pi{}}{4}$
  \item $\dfrac{4\pi{}}{3}$
  \item $\dfrac{7\pi{}}{10}$
  \end{colenumerate}
\end{exercice*}
\begin{corrige}
  \begin{enumerate}
  \item $\dfrac{3\pi }{5}$ car ${-{\dfrac{\text{7$\pi
            $}}{5}}}\notin \left]-\pi \ ;\ \pi \right]$ mais 
    ${-{\dfrac{\text{7$\pi $}}{5}}+2\pi =\dfrac{3\pi }{5}}\in \left]-\pi
      \ ;\ \pi \right]$

  \item $\dfrac{\pi }{2}$ car ${\dfrac{\text{18}\pi }{4}}\notin
    \left]-\pi \ ;\ \pi \right]$ mais ${\dfrac{\text{18}\pi }{4}}-4\pi
    =\dfrac{\pi }{2}\in \left]-\pi \ ;\ \pi \right]$

  \item $-\dfrac{2\pi}{3}$ car $\dfrac{4\pi}{3}\notin \left]-\pi \ ;\ \pi
    \right]$ mais $\dfrac{4\pi}{3}-2\pi =-\dfrac{2\pi}{3}\in \left]-\pi
      \ ;\ \pi \right]$

  \item $\dfrac{7\pi}{10}$ car $\dfrac{7\pi}{10}\in \left]-\pi \ ;\ \pi
    \right]$
  \end{enumerate}
\end{corrige}
