3.在直角三角形ABC中,∠ACB=90°,直線l過點(diǎn)C.
(1)當(dāng)AC=BC時(shí),如圖1,分別過點(diǎn)A和B作AD⊥直線l于點(diǎn)D,BE⊥直線l于點(diǎn)E.求證:DE=AD+BE;
(2)當(dāng)AC=8cm,BC=6cm時(shí),如圖2,BF⊥直線l于點(diǎn)K,且BK=FK,連接CF.點(diǎn)M從A點(diǎn)出發(fā),以每秒1cm的速度沿A→C路徑運(yùn)動(dòng),終點(diǎn)為C,點(diǎn)N以每秒3cm的速度沿F→C→B→C→F路徑運(yùn)動(dòng),終點(diǎn)為F,分別過點(diǎn)M、N作MD⊥直線l于點(diǎn)D,NE⊥直線l于點(diǎn)E,點(diǎn)M、N同時(shí)開始運(yùn)動(dòng),各自達(dá)到相應(yīng)的終點(diǎn)時(shí)停止運(yùn)動(dòng),設(shè)運(yùn)動(dòng)時(shí)間為t秒.當(dāng)△MDC與△CEN全等時(shí),求t的值.