\(M=-\left(2x+1+2\sqrt{x\left(x+1\right)}-x-2\right)\left(2x+1-2\sqrt{x\left(x+1\right)}-x-2\right)\)
\(=-\left(x-1+2\sqrt{x^2+x}\right)\left(x-1-2\sqrt{x^2+x}\right)\)
\(=-\left(x-1\right)^2+4\left(x^2+x\right)\)
\(=3x^2+6x-1\)
\(\Rightarrow\)M là số tự nhiên