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Étudier l’existence de solutions du système : \[\left\{\begin{array}{ccccccccc} 4x &+&3y &+&2z &+& t &=& a \cr x &+&4y &+&3z &+&2t &=& b \cr 2x &+& y &+&4z &+&3t &=& c \cr 3x &+&2y &+& z &+&4t &=& d.\cr\end{array}\right.\]
Étudier l’existence de solutions du système : \[\left\{\begin{array}{ccccccccc} 4x &+&3y &+&2z &+& t &=& a \cr x &+&4y &+&3z &+&2t &=& b \cr 2x &+& y &+&4z &+&3t &=& c \cr 3x &+&2y &+& z &+&4t &=& d.\cr\end{array}\right.\]