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$$ \displaylines { \text{pentru} \ f_2 = x_2, \ x_1 \to \infty \ \text{avem:} \\ - \frac{n_2}{f_2} = \frac{n_2 - n_1}{R} \implies f_2 = \frac{n_2 R}{n_2 - n_1} } $$