Câu 4:

Giải hệ phương trình tuyến tính $\left\{ \begin{array}{l}\mathop {2x}\nolimits_1 + 7\mathop x\nolimits_2 + 3\mathop x\nolimits_3 + \mathop x\nolimits_4 = 5\\\mathop x\nolimits_1 + 3\mathop x\nolimits_2 + 5\mathop x\nolimits_3 - 2\mathop x\nolimits_4 = 3\\\mathop x\nolimits_1 + \mathop {5x}\nolimits_2 - 9\mathop x\nolimits_3 + 8\mathop x\nolimits_4 = 1\\5\mathop x\nolimits_1 + 18\mathop x\nolimits_2 + \mathop {4x}\nolimits_3 + 5\mathop x\nolimits_4 = 12\end{array} \right.$