①求DE的長(zhǎng);
②動(dòng)點(diǎn)M從點(diǎn)F出發(fā)沿FC向C點(diǎn)勻速運(yùn)動(dòng),速度為每秒2個(gè)單位長(zhǎng)度,同時(shí)動(dòng)點(diǎn)N以每秒1個(gè)單位長(zhǎng)度的速度從C點(diǎn)出發(fā),沿CB勻速向B點(diǎn)運(yùn)動(dòng),當(dāng)點(diǎn)M到達(dá)C點(diǎn)時(shí),兩點(diǎn)同時(shí)停止運(yùn)動(dòng),設(shè)運(yùn)動(dòng)時(shí)間為t,若以M,N,C為頂點(diǎn)的三角形與相似時(shí),求t的值.