— Hi, Mike. What time do you usually get up _______ weekends?
— I usually get up _______ about eight o'clock.
A:on; at
B:in; at
C:on; in
圆外切等腰梯形一腰长为5cm,则梯形的中位线长为( )
如图,在正方体ABCD﹣A1B1C1D1中,点M,N分别是面对角线A1B与B1D1的中点,若DA→=a→,DC→=b→,DD1→=c→,则MN→=( )
如图为远距离的简化电路图.发电厂的输出电压是U,用等效总电阻是r的两条输电线输电,输电线路中的电流是I1,其末端间的电压为U1.在输电线与用户间连有一理想变压器,流入用户端的电流为I2.则( )