magnitude是精确距离，sqrMagnitude是节省CPU的粗略距离，精度有损失
CubeA坐标 x:0 y:0.844 z:0
CubeC坐标 x:0 y:0 z:0
Vector3 aaa = A.position - C.position;
Debug.DrawLine(A.position, C.position, Color.green);
Debug.Log("aaa: " + aaa.ToString() + " magnitude:" + aaa.magnitude + " sqrMagnitude:" + aaa.sqrMagnitude);
