[tex]dane:\\s_1 = s_2 = s = 60 \ km\\v_1 = 90\frac{km}{h}\\v_2 = 60\frac{km}{h}\\szukane:\\v_{sr} = ?\\\\Rozwiazanie\\\\v_{sr} = \frac{2s}{t} = \frac{2s}{\frac{s}{v_1}+\frac{s}{v_2}} =\frac{2}{\frac{1}{v_1}+\frac{1}{v_2}} = \frac{2}{\frac{v_1+v_2}{v_1v_2}} = \frac{2v_1v_2}{v_1+v_2}[/tex]
[tex]v_{sr} = \frac{2\cdot90\cdot60}{90+60} = 72\frac{km}{h}\\\\Odp. \ C.[/tex]
