Ta có $y=\left( 2x+1 \right){{e}^{\frac{x}{2}}}\Rightarrow y'=2{{e}^{\frac{x}{2}}}+\frac{1}{2}\left( 2x+1 \right).{{e}^{\frac{x}{2}}}\Rightarrow y'-y\ne {{e}^{x}}$

Xét $y=\left( x+1 \right){{e}^{x}}\Rightarrow y'={{e}^{x}}+\left( x+1 \right){{e}^{x}}\Rightarrow y'={{e}^{x}}+y$

Xét $y=2{{e}^{x}}+1\Rightarrow y'=2{{e}^{x}}=y-1$

Xét $y=x{{e}^{-x}}\Rightarrow y'={{e}^{-x}}-x{{e}^{-x}}={{e}^{-x}}-y$ .