a)
[tex]dane:\\s_1 =s_2= s\\v_1 = 90\frac{km}{h}\\s_2 = s\\v_2 = 50\frac{km}{h}\\szukane:\\v_{sr} = ?\\\\v_{sr} = \frac{s_{c}}{t_{c}}\\\\s_{c}= s_1 + s_2 = s+s = 2\\\\t = \frac{s}{v}\\\\t_{c}= t_1+t_2 = \fracPs}{90} + \frac{s}{50} \ [h] = \frac{5s+9s}{450} = \frac{14s}{450} \ [h]\\\\v_{sr} = \frac{2s}{\frac{14s}{450}}=\frac{450}{7} \ \frac{km}{h}\\\\\boxed{v_{sr} = 64,3\frac{km}{h}}[/tex]
b)
[tex]dane:\\t_1 = t_2 = t\\v_1 = 90\frac{km}{h}\\v_2 = 50\frac{km}{h}\\szukane:\\v_{sr}=?\\\\v_{sr} = \frac{s_{c}}{t_{c}}\\\\s_{c} = v_1\cdot t+v_2\cdot t\\\\t_{c} = 2t\\\\v_{sr} = \frac{v_1\cdot t+v_2\cdot t}{2t}\\\\v_{sr} = \frac{t(v_1+v_2)}{2t}\\\\v_{sr} = \frac{v_1+v_2}{2}\\\\v_{sr} = \frac{90+50}{2} \ [\frac{km}{h}]\\\\\boxed{v_{sr} = 70\frac{km}{h}}[/tex]