Question

(x+y+z)=x²+y²+z²+2xy+2yz+2xz

Answer 1

yes this is an identity of square of three variables

this can be checked simply by inserting the values

Answer 2

(x+y+z)² = (x+y +z)(x+y +z)
=> (x+y+z)² =x²+y²+z²+2xy+2yz+2xz