a) co ab//cd//mn ( gt) hay eb//fc//on va ae//mo//df
tu giac ebcf co eb//fc
=> tu giac ebcf la hinh thang co
on// eb// fc ( cmt)
oe=of ( gt)
=> oe=of (dl)
=> n la trung diem bc
cmtt co m la trung diem ad
b) tu giac ebcf co
oe = of ( gt)
nb= nc (cmt)
=> on la duong trung binh ( dn)
=> eb+ cf)/2 ( tc)
cmtt co om = ( ea + df) /2
maf eb= ea, cf=df (gt)
=> om= on