Children often make mistakes spelling.
A:in
B:for
C:on
D:at
双曲线 x29−y2 =1有动点P,F1,F2是曲线的两个焦点,则△PF1F2的重心M的轨迹方程为____.
如图所示,用滑轮组把一个物体匀速拉起,不计摩擦,则下列 说法中正确的是( )