... y 2 ) 2 = x 2 1+ y 2 1+ x 2 2+ y 2 2− (2x1x 2 +2y1y 2 ) =2( x 2 1+ y 2 1) +2( x 2 2+ y 2 2) − [(x1+ x 2 ) 2 +(y1+ y 2 ) 2 ] =2| z1| 2 +2| z 2 | 2 − 2| z1+ z 2 | 2 .Nhu.ng z1+ z 2 = ... iy1, z 2 = x 2 + iy 2 . Khi d´oz1+ z 2 = x1+ x 2 + i(y1+ y 2 ),z1− z 2 = x1− x 2 + i(y1− y 2 ),|z1+ z 2 | 2 =(x1+ x 2 ) 2 +(y1+ y 2 ) 2 ,|z1− z 2 | 2 =(x1− x 2 ) 2 +(y1− ... z 2 | 2 =(x1− x 2 ) 2 +(y1− y 2 ) 2 .T`u.d´othudu.o..c|z1+ z 2 | 2 + |z1− z 2 | 2 =2( x 2 1+ y1) 2 +2( x 2 2+ y 2 2) =2( |z1| 2 + |z 2 | 2 ).T`u.hˆe.th´u.cd˜a c h...