【導(dǎo)讀】ABCD中,BD、AC相交于O,AC=6,AB=3,則BC=_________,BD=_________,是_________的平行四邊形是矩形.ABCD的兩條對(duì)角線交于點(diǎn)O,,AC=2BC,則∠AOB=_________.的四邊形四邊中點(diǎn)所得四邊形是矩形,順次連結(jié)_________的四邊形四邊中點(diǎn)所得四邊形是菱形.即新四邊形的形狀與原四邊形的_________有關(guān).1∶2,則這兩個(gè)角的度數(shù)分別是_________.ABCD的兩條對(duì)角線長(zhǎng)分別是6cm和8cm,則菱形的周長(zhǎng)是_________.ABCD中,AD=9cm,AB=3cm,將其折疊使點(diǎn)D與點(diǎn)B重合,求折疊后DE的長(zhǎng).°矩形菱形對(duì)角線互相垂直對(duì)角線相等對(duì)角線cm°,120°