X1-X2-X3-2X4=-1,

X1+X2-2X3+X4= 1，

X1+X2 =2，

+X2 +X3-X4 =1

D=

| 1 -1 -1 -2 |

| 1 1 -2 1 |

| 1 1 0 0 |

| 0 1 1 -1 |

=

| 1 -1 -1 -2 |

| 0 2 -1 3 |

| 0 2 1 2 |

| 0 1 1 -1 |

=

| 2 -1 3 |

| 2 1 2 |

| 1 1 -1 |

= ...

= -7

D1=

|-1 -1 -1 -2 |

| 1 1 -2 1 |

| 2 1 0 0 |

| 1 1 1 -1 |

= ...

= -9

D2=

| 1 -1 -1 -2 |

| 1 1 -2 1 |

| 1 2 0 0 |

| 0 1 1 -1 |

= ...

= -5

D3=

| 1 -1 -1 -2 |

| 1 1 1 1 |

| 1 1 2 0 |

| 0 1 1 -1 |

=...

= -5

D4=

| 1 -1 -1 -1 |

| 1 1 -2 1 |

| 1 1 0 2 |

| 0 1 1 1 |

= ...

= -3

x1=D1/D=9/7

x2=D2/D=5/7

x3=D3/D=5/7

x4=D4/D=3/7