[tex]Dane:\\x = 20 \ cm\\y = 30 \ cm\\Szukane:\\f = ?\\\\\frac{1}{f} = \frac{1}{x}-\frac{1}{y} \ - \ rownanie \ soczewki \ dla \ obrazu \ pozornego\\\\\frac{1}{f} = \frac{y-x}{xy}\\\\f = \frac{xy}{y-x} = \frac{20 \ cm\cdot30 \ cm}{30 \ cm - 20 \ cm} = \frac{600 \ cm^{2}}{10 \ cm}\\\\\boxed{f = 60 \ cm}[/tex]
Odp. Ogniskowa soczewki ma długość 60 cm.
