BA=B+2E
BA-B=2E
B(A-E)=2E
B(1 1
-1 1)
=(2 0
0 2)
取行列式,得
|B|*|1 1
-1 1|
=|2 0
0 2|
(1×1-（-1）×1)|B|=2×2
2|B|=4
|B|=2