如图所示，物体的重量是600N，不计动滑轮和绳子的摩擦和滑轮重时，当匀速拉动绳子时，作用在绳子上的拉力F为_________________N。
200
若
，A=αβ1，B是4阶矩阵，r(B) =2，则r(AB-2B)=______．
is known to us all is that the old worker, life was hard in the past, still works hard in his seventies.
A．As; whom
B．What; whom
C．It; whose
D．What; whose
，A=αβ1，B是4阶矩阵，r(B) =2，则r(AB-2B)=______．