[tex]Dane:\\x = 120 \ cm\\y = 30 \ cm\\Szukane:\\f = ?\\\\Rozwiazanie\\\\\frac{1}{f} = \frac{1}{x}+\frac{1}{y}\\\\\frac{1}{f} = \frac{x+y}{xy}\\\\f = \frac{xy}{x+y} = \frac{120 \ cm\cdot30 \ cm}{120 \ cm+30 \ cm} = \frac{3600 \ cm^{2}}{150 \ cm}\\\\\boxed{f = 24 \ cm}[/tex]
Odp. Długość ogniskowej soczewki f = 24 cm.
