Because e and f are the midpoint of BC and AD respectively, that is, AF = DF and CE = BE.
Because AF = FD and DK = BK.
So FK is the center line of triangle ABD, which means FK∨AB? 2FK=AB (think for yourself)
So what? ∠ 1=∠4(。 . )
Similarly, because ce = be, bk = dk? Get EK∨CD 2EK=CD
So ∠2=∠3 (. . )
Because AB=CD
So FK=EK
So < 1 = < 2.
Get ∠3=∠4.
So the triangle OMN is an isosceles triangle.