2) \(x^2-y^2-x-y=x\left(x-1\right)-y\left(y-1\right)\)
=> Chịu sai rồi <<
5) \(\left(2x+1\right)^2-\left(x-1\right)^2=\left[\left(2x+1\right)-\left(x-1\right)\right].\left[\left(2x+1\right)+\left(x-1\right)\right]\)
3) \(x^2-5x+5y+y^2=x\left(x-5\right)+y\left(5+y\right)\)
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