B^2=(-B^T)(-B^T)=(B^T)^2=(B^2)^T,说明B^2为对称矩阵
(AB-BA)^T=(AB)^T-(BA)^T=(B^T)(A^T)-(A^T)(B^T)=(-BA)-(-AB)=-BA+AB,即AB-BA,这说明AB-BA也是对称矩阵