Răspuns :
[tex]=(x+y-z)(x+y-z)-( x^{2} +2xy+ y^{2})-( y^{2} +2zy+ z^{2})= [/tex]
[tex]x^{2} +xy-xz+xy+y ^{2}-yz-xz-yz+ z^{2} - x^{2} -2xy- y^{2}- y^{2}-2yz- z^{2} [/tex]
=-2xz-y²-4yz
[tex]x^{2} +xy-xz+xy+y ^{2}-yz-xz-yz+ z^{2} - x^{2} -2xy- y^{2}- y^{2}-2yz- z^{2} [/tex]
=-2xz-y²-4yz