(X+y)^2=x^+2xy+y^2
(X-y)^2=x^2-2xy+y^2
X^2+2xy+y^2=x^2-2xy+y^2+4xy
X^2+y^2=x^2+y^2 -4xy +4xy
Se simplifica -4xy cu 4xy si ramane
X^2+y^2=x^2+y^2
^-la patrat
a-b=2
ab=4
(a-b)^2=a^2-2ab+b^2
4^2=a^2-2•4+b^2
16=a^2+b^2-8
a^2+b^2=8
(a+b)^2=a^2+2ab+b^2
(a+b)^2= 8+ 2•4
(a+b)^2=8+8
(a+b)^2=16
a+b=+ - 4
