## Precalculus (6th Edition) Blitzer

$f_{2}= \displaystyle \frac{ff_{1}}{f_{1}-f}, \quad$focal length of glasses
$f=\displaystyle \frac{f_{1}f_{2}}{f_{1}+f_{2}}$ ... multiply with the LCD, $(f_{1}+f_{2})$ $f(f_{1}+f_{2})=f_{1}f_{2}$ $ff_{1}+ff_{2}=f_{1}f_{2}$ ... transfer $f_{1}f_{2}$ to the LHS: add $-f_{1}f_{2}-ff_{1}$ $ff_{2}-f_{1}f_{2}=-ff_{1}\quad$ ... factor out $f_{2}$ $f_{2}(f-f_{1})=-ff_{1}\quad$ ... divide with $(f-f_{1})$ $f_{2}=\displaystyle \frac{-ff_{1}}{f-f_{1}}$ $f_{2}= \displaystyle \frac{ff_{1}}{f_{1}-f}$ (see example 5) focal length of glasses, $f_{1}=$ distance from the lens $f_{2}=$ distance from the lens to your retina