21.已知,矩形ABCD中,AB=4 cm,BC=8 cm,AC的垂直平分線EF分別交AD,BC于點(diǎn)E、F,垂足為O.
(1)如圖1,連接AF、CE,求證四邊形AFCE為菱形,并求AF的長(zhǎng);
(2)如圖2,設(shè)點(diǎn)G是EF上任意一點(diǎn),GM⊥BC于點(diǎn)M,GN⊥EC于點(diǎn)N,求GM+GN的值;
(3)如圖3,動(dòng)點(diǎn)P、Q分別從AC兩點(diǎn)同時(shí)出發(fā),沿△AFB和△CDE各邊勻速運(yùn)動(dòng)一周,即點(diǎn)P自A→F→B→A停止,點(diǎn)Q自C→D→E→C停止,在運(yùn)動(dòng)過程中:
①已知點(diǎn)P的速度為每秒5 cm,點(diǎn)Q的速度為每秒4 cm,運(yùn)動(dòng)時(shí)間為x秒,當(dāng)A、C、P、Q四點(diǎn)為頂點(diǎn)的四邊形是平行四邊時(shí),求x的值.
②若點(diǎn)P、Q的運(yùn)動(dòng)路程分別為a,b(單位:cm,ab≠0),已知A、C、P、Q四點(diǎn)為頂點(diǎn)的四邊形是平行四邊形,求a與b滿足的數(shù)量關(guān)系式.