Cambio di variabile. Poniamo
$ = \displaystyle\lim_{y \to +\infty} (1+\frac{1}{y})^{2y+3} = $
$ = \displaystyle\lim_{y \to +\infty} (1+\frac{1}{y})^{2y} \cdot (1+\frac{1}{y})^3 = $
$ = \displaystyle\lim_{y \to +\infty} [(1+\frac{1}{y})^y]^2 \cdot (1+\frac{1}{y})^3 = e^2 \cdot 1 = e^2$