70÷8=____……____
① mm−n + nn−m
② a2−b2ab ÷(a﹣b)2
③( xx−2 ﹣ xx+2 )÷ 4xx−2 .
September is ____________ month of the year.
如图所示的电路,电压U为24V,电流表的示数为1.2A,电压表的示数为6V,则电阻R1阻值为____ Ω.