Câu 3:

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