[tex] \frac{1}{f} = \frac{1}{x1} + \frac{1}{x2} [/tex]
x1= distanta obiect-lentila
x2= distanta imagine-lentila
[tex] \frac{1}{f} = \frac{1}{50} + \frac{1}{20} [/tex]
[tex] \frac{1}{f} = \frac{7}{100} [/tex]
f=[tex] \frac{100}{7} =14,(285714) cm[/tex]
