The Class Notes

$Limit\ A\ \to \infty :$

$X_{0}\ ={\frac {1}{\beta }}X_{s}\ \Rightarrow V_{0}={\frac {R_{1}+R_{2}}{R_{1}}}V_{in}$

$X_{i}\ =0$

Drawing

$V_{in}\ =V_{0}({\frac {R_{1}}{R_{1}+R_{2}}})$