↑AB = ↑DC =↑ b
↑BC = ↑AD = ↑ a
↑AM = ↑AB + 3/4 ↑ BC = ↑ AB + 3/4 ↑AD = ↑b +3/4 ↑a
↑MD=-↑AM +↑AD = -(↑b +3/4 ↑a) +a = 1/4↑a -↑b
или
↑MD=↑MC +↑CD=↑MC -↑DC =1/4↑a -↑b
↑AB = ↑DC =↑ b
↑BC = ↑AD = ↑ a
↑AM = ↑AB + 3/4 ↑ BC = ↑ AB + 3/4 ↑AD = ↑b +3/4 ↑a
↑MD=-↑AM +↑AD = -(↑b +3/4 ↑a) +a = 1/4↑a -↑b
или
↑MD=↑MC +↑CD=↑MC -↑DC =1/4↑a -↑b