factorization
x2+2xy+y2-9
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[tex]\\ \sf\longmapsto x^2+2xy+y^2-9[/tex]
[tex]\boxed{\sf (a+b)^2=a^2+b^2+2ab}[/tex]
[tex]\\ \sf\longmapsto (x+y)^2-9[/tex]
[tex]\\ \sf\longmapsto (x+y)^2-3^2[/tex]