x2+y2+z2−xy−yz−zx=12[(x−y)2+(y−z)2+(z−x)2]
First take R.H.S
12[(x−y)2+(y−z)2+(z−x)2]
We know that (x−y)2=x2−2xy+y2
Therefore, 12[x2−2xy+y2+y2−2yz+z2+z2−2zx+x2]
= 12[2x2+2y2+2z2−2xy−2yz−2zx]
Taking 2 as common
= 12×2[x2+y2+z2−xy−yz−zx]
= x2+y2+z2−xy−yz−zx
So, L.H.S = R.H.S
x2+y2+z2−xy−yz−zx=x2+y2+z2−xy−yz−zx
Hence x2+y2+z2−xy−yz−zx=12[(x−y)2+(y−z)2+(z−x)2] is proved.