Calculati (u+r-s)²
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se calculeaz dupa formula invatata tinand cont de regula semnelor
si ca
-s= +(-s)
 atunci avem
 u²+r²+(-s)²+2ur+2u*(-s)+2r*(-s)=u²+r²+s²+2ur-2us-2rs
de obicei nu se mai trece prin faza a doua, se inmultesc direct, tinand cont de regula semnelor la inmultire

   
[tex]\displaystyle \bf\\ (u+r-s)^2 = (u+r-s)(u+r-s)=\\\\ \underbrace{\bf u^2 + ur - us} + \underbrace{\bf ur + r^2 - rs} \underbrace{\bf -us-rs+s^2}=\\\\ = u^2+r^2+s^2 +2ur - 2us - 2rs =\\\\ = \boxed{\bf u^2+r^2+s^2 +2(ur - us - rs) }[/tex]