We have,
M and N are the midpoints of the diagonals AC and BD$ respectively.
Let
→A=→a
→B=→b
→C=→c
→D=→d
Now,
−→M=→a+→c2
→N=→b+→d2
L.H.S.
−−→AB+−−→AD+−−→CB+−−→CD=4−−−→MN
⇒→b−→a+→d−→a+→b−→c+→d−→c
⇒2→d−2→a+2→b−2→c
⇒2(→b+→d)−2(→a+→c)
⇒4N−4M
⇒4(N−M)
⇒4MN
R.H.S.
Hence, proved.