4 4
7 8
B=
1 1 1
0 1 1
0 0 1
Then a- 1 =
2 - 1
-7/4 1
B^- 1=
1 - 1 0
0 1 - 1
The original matrix is divided into blocks.
O A
B O
The inverse matrix is:
Oh, b-1
A-1island
0 0 1 - 1 0
0 0 0 1 - 1
0 0 0 0 1
2 - 1 0 0 0
-7/4 1 0 0 0