The point z1,z2,z3,z4 in the complex plane are the vertices of a parallelogram taken in order if and only if
z1+z4=z2+z3
z1+z3=z2+z4
z1+z2=z3+z4
none of these
Diagonals of a parallelogram are bisect each other.
z1+z32=z2+z42⇒z1+z3=z2+z4