梯形中,AD//EF//MN//BC,AE=EM=1/3MB.1.设AD=a,EF=a+b,求MN和BC的长.2.若AD=3Ccm_数学解答

梯形中,AD//EF//MN//BC,AE=EM=1/3MB.1.设AD=a,EF=a+b,求MN和BC的长.2.若AD=3Ccm,BC=8cm,求EF和MN的长.
过程啊,

人气：475 ℃　时间：2020-03-28 07:08:02

解答

1.作DP // AB交EF,MN于E',M'E'F = EF - EE' = EF - AD = a+b - a = bDF = FN => M'N = 2E'F = 2b=> MN = MM'+M'N = a+2bDC = 5DF => B'C=5E'F = 5b=> BC=a+5b2.BC = AD+5b = 3+5b=8,b=1EF = 3+1 = 4MN = 3+2 = 5