## Knowledge#

### 矩阵乘法#

It is my experience that proofs involving matrices can be shortened by 50% if one throws the matrices out. - Emil Artin

### More thing#

1. $(AB)^{-1}=B^{-1}A^{-1}$ .

从右往左，先结合$B$变换再结合$A$变换 的逆变换，想把这个过程变回去，要考虑坐标系不动，所以要先作$A$变换的逆变换再做$B$变换的逆变换才可以。