fie zi=ai+bi
deci /zi/2=ai2+bi2
/zi+zj/2=(ai+aj)2+(bi+bj)2
/z1+z2+z3/2=(a1+a2+a3)2+(b1+b2+b3)2
inlocuind si separand a de b ar trebui sa demonstram
a12+a22+a32+(a1+a2+a3)2=(a1+a2)2+(a1+a3)2+(a2+a3)2
adica
a12+a22+a32+a12+a22+a32+2a1a2+2a1a3+2a2a3=
a12+a22+2a1a2+a12+a32+2a1a3+a22+a32+2a2a3
care este adevarata
la fel pt b uri
deci si pentru z uri