A=
1 -1 1 -1
-1 1 1 -2
1 1 1 0
-1 -2 0 -2
r2+r1,r3-r1,r4+r1 然后对列作同样的变换
1 0 0 0
0 0 2 -3
0 2 0 1
0 -3 1 -3
r2-r3-r4
1 0 0 0
0 1 1 -1
0 1 0 1
0 -1 1 -3
r3-r2,r4+r2
1 0 0 0
0 1 0 0
0 0 -1 2
0 0 2 -4
r4+2r3
1 0 0 0
0 1 0 0
0 0 -1 0
0 0 0 0
所以标准形为 y1^2+y2^2-y3^2.