The Eiffel Towel, a cultural icon of France, was completed at the end of the ____ (nineteen) century.
如果a与-1互为相反数,则a+2等于( )
点电荷A和B,分别带正电和负电,电量分别为4Q和Q,在AB连线上,如图所示,电场强度为零的地方在( )
如图,在菱形ABCD中,∠A=45°,DE⊥AB,垂足为E,若CD=4cm,则菱形ABCD的面积是____ cm2.