1 -1 0 0
0 1 -1 0
0 0 1 -1
a b c d
= (第四行+第一行×(-a))
1 -1 0 0
0 1 -1 0
0 0 1 -1
0 a+b c d
=(第四行+第二行×(-a-b))
1 -1 0 0
0 1 -1 0
0 0 1 -1
0 0 a+b+c d
=(按第一列展开)
1 -1 0
0 1 -1
0 a+b+c d
=(按第一列展开)
1 -1
a+b+c d
=d+a+b+c
1 -1 0 0
0 1 -1 0
0 0 1 -1
a b c d
= (第四行+第一行×(-a))
1 -1 0 0
0 1 -1 0
0 0 1 -1
0 a+b c d
=(第四行+第二行×(-a-b))
1 -1 0 0
0 1 -1 0
0 0 1 -1
0 0 a+b+c d
=(按第一列展开)
1 -1 0
0 1 -1
0 a+b+c d
=(按第一列展开)
1 -1
a+b+c d
=d+a+b+c