Câu 20:
Giải hệ phương trình tuyến tính {4x1+3x2+x3+5x4=7x1−2x2−2x3−3x4=33x1−x2+2x3=−12x1+3x2+2x3−8x4=−7\left\{ \begin{array}{l}4\mathop x\nolimits_1 + 3\mathop x\nolimits_2 + \mathop x\nolimits_3 + 5\mathop x\nolimits_4 = 7\\\mathop x\nolimits_1 - 2\mathop x\nolimits_2 - 2\mathop x\nolimits_3 - 3\mathop x\nolimits_4 = 3\\3\mathop x\nolimits_1 - \mathop x\nolimits_2 + 2\mathop x\nolimits_3 = - 1\\2\mathop x\nolimits_1 + 3\mathop x\nolimits_2 + 2\mathop x\nolimits_3 - 8\mathop x\nolimits_4 = - 7\end{array} \right.⎩⎨⎧4x1+3x2+x3+5x4=7x1−2x2−2x3−3x4=33x1−x2+2x3=−12x1+3x2+2x3−8x4=−7
