箭头略去,AC指向量AC,AC=BC-BA,BD=BC+BA
‖AC‖^2==‖BA‖^2+‖BC‖^2-2
‖BD‖^2==‖BA‖^2+‖BC‖^2+2
因此‖AC‖^2+‖BD‖^2=2(‖BA‖^2+‖BC‖^2)