∵EF∥AB
∴△DEM∽△DAB,△CFN∽△CBA
∴EM/AB=DM/DB,FN/BA=CF/CB
∵ABCD是梯形
∴AB∥CD
那么EF∥CD
∴△BMF∽△BDC
∴BM/BD=BF/BC
1-BM/BD=1-BF/BC
即DM/BD=CF/BC
又EM/AB=DM/DB,FN/BA=CF/CB
得EM/AB=FN/BA
∴EM=FN
∵EF∥AB
∴△DEM∽△DAB,△CFN∽△CBA
∴EM/AB=DM/DB,FN/BA=CF/CB
∵ABCD是梯形
∴AB∥CD
那么EF∥CD
∴△BMF∽△BDC
∴BM/BD=BF/BC
1-BM/BD=1-BF/BC
即DM/BD=CF/BC
又EM/AB=DM/DB,FN/BA=CF/CB
得EM/AB=FN/BA
∴EM=FN