Bài 1:
a. \(\left\{{}\begin{matrix}v_{tb1}=\dfrac{s_1+s_2}{t_1+t_2}=\dfrac{s}{\dfrac{\dfrac{1}{2}s}{20}+\dfrac{\dfrac{1}{2}s}{60}}=\dfrac{s}{\dfrac{1}{40}s+\dfrac{1}{120}s}=30\left(\dfrac{km}{h}\right)\\v_{tb2}=\dfrac{s_1+s_2}{t_1+t_2}=\dfrac{20\cdot\dfrac{1}{2}t+60\cdot\dfrac{1}{2}t}{t}=\dfrac{10t+30t}{t}=40\left(\dfrac{km}{h}\right)\end{matrix}\right.\)
b. Theo đề bài, ta có: \(t_1-t_2=0,5\)
\(\Leftrightarrow\dfrac{s}{30}-\dfrac{s}{40}=0,5\)
\(\Leftrightarrow s=60km\)
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