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Étudier l’existence de solutions du système : \[\left\{\begin{array}{ccccccc} x\cos2\alpha &+& y\cos\alpha &+& z &=& a \cr x\cos2\beta &+& y\cos\beta &+& z &=& b \cr x\cos2\gamma &+& y\cos\gamma &+& z &=& c.\cr\end{array}\right.\]
Étudier l’existence de solutions du système : \[\left\{\begin{array}{ccccccc} x\cos2\alpha &+& y\cos\alpha &+& z &=& a \cr x\cos2\beta &+& y\cos\beta &+& z &=& b \cr x\cos2\gamma &+& y\cos\gamma &+& z &=& c.\cr\end{array}\right.\]