\magnification=\magstep2

$$
{ {1} \over {f} } = (n - 1) 
        {\left( {{1}\over{r_1}} - {{1}\over{r_2}} \right) }
$$


$$
{ {1} \over {f} } = (n - 1) 
        {\left( {{2}\over{r}} \right) }
$$


$$
{ {1} \over {f} } = {\left( {{1}\over{o}} + {{1}\over{i}} \right) }
$$

\bye
