Giải:
a) Trong
\(\Delta {t_1} = 10\min \left\{ \matrix{ \Delta {x_1} = 2,5km \hfill \cr {v_{t{b_1}}} = {{\Delta {x_1}} \over {\Delta {t_1}}} = {{2,5} \over {10}} = 0,25(km/\min ) \hfill \cr} \right.\)
b) Trong \(\Delta {t_2} = {t_2} - {t_1} = 20 - 10 = 10\min \)
\(\left\{ \matrix{ \Delta {x_2} = 4,0 - 2,5 = 1,5(km) \hfill \cr {v_{t{b_2}}} = {{\Delta {x_2}} \over {\Delta {t_2}}} = {{1,5} \over {10}} = 0,15(km/\min ) \hfill \cr} \right.\)
c) Trong
\(\Delta {t_1} = 30\min :\left\{ \matrix{ \Delta x = 4,5km \hfill \cr {v_{tb}} = {{\Delta x} \over {\Delta t}} = {{4,5} \over {30}} = 0,15(km/\min ) \hfill \cr} \right.\)