1x+x2-2x3+x1=4, ② 2 x3+x4=0, B ③+52 2x4=-6, ④-3② x,=-3 「x1+x2-2x3+x=4,① ③>④ x2-x3+x4=0, (B4) ④-2 =-3. 0=0 ④ 用“回代”的方法求出解: 上页( ) B3 ( ) B4        = − = − − + = + − + = 3, 2 6, 0, 2 4, 4 4 2 3 4 1 2 3 4 x x x x x x x x x 1 3 4 2 + 5 2 2 1  3 4 − 3 2 2        = = − − + = + − + = 0 0, 3, 0, 2 4, 4 2 3 4 1 2 3 4 x x x x x x x x 1 3 4 3  2 4 − 2 4 3 用“回代”的方法求出解：