طبق فرض داريم:  ${{(A-B)}^{2}}={{A}^{2}}+{{B}^{2}}\Rightarrow (A-B)(A-B)={{A}^{2}}+{{B}^{2}}\Rightarrow {{A}^{2}}-AB-BA+{{B}^{2}}={{A}^{2}}+{{B}^{2}}\Rightarrow -AB-BA=\overline{O}\Rightarrow AB=-BA$