Ta co:
\(x^2+x-2011.2012=x^2+x-\left(2012-1\right).2012\)
\(=x^2+x-2012^2+2012=\left(x^2-2012^2\right)+\left(x+2012\right)\) \(=\left(x+2012\right)\left(x-2012\right)+\left(x+2012\right)=\left(x+2012\right)\left(x-2012+1\right)\)
\(=\left(x+2012\right)\left(x-2011\right)\)