[tex]v_{sr}=\frac{s_1+s_2}{t_1+t_2},\ gdzie\ s_1=s_2\\\\ v_{sr}=\frac{2s}{t_1+t_2}\\\\ v=\frac{s}{t}\Rightarrow t=\frac{s}{v}\\\\ v_{sr}=\frac{2s}{\frac{s}{v_1}+\frac{s}{v_2}}\\\\ v_{sr}=\frac{2}{\frac{1}{v_1}+\frac{1}{v_2}}\\\\ v_{sr}=\frac{2v_1*v_2}{v_1+v_2}\\\\[/tex]
