Question

prove that : x²+y²+z²-xy-yz-zx is always positive

Answer 1

Its not in all cases, if you take x=1,y=1,z=1

This equation will give “0”.

You can't say Always Positive.

Even this is valid for all equal value of x,y,z

I.e. x=y=z and should belong to Real No. set

[ 0 is neither positive nor negeative ]