证明:四边形ABCD是正方形, ∠A=∠B=∠C=∠D=90° AB= BC= CD=DA ∴A"B"= B"C" = C"D"= D"A"= 又AA"= BB"=DD" ∴A"B = B"C= C"D=D"A ∴A"B"= B"C"= CD"=D"A" ∴四边形A"B"C"D"为菱形 ∴AA"= BB",AD"=A"B,A"D"=A"B" ∴∠D"A"A =∠A"B"B ∴.△AA"D"≌△BB"A" ∴∠D"A"A =∠A"B"B ∴∠A"B"B +∠BA"B "=90° ∴∠D"A"A+BA"B"=90° ∴∠D"A"B"=90° ∴ 四边形A"B"C"D"是正方形。 |