Bài 24:
Ta có: \(v_{tb}=\dfrac{S}{T}=\dfrac{s+s'}{T}\)
\(\Leftrightarrow v_{tb}=\dfrac{vt+v't'}{t}=\dfrac{v\cdot\dfrac{t}{2}+v'\cdot\dfrac{t}{2}}{T}\)
\(\Leftrightarrow30=\dfrac{55\cdot\dfrac{t}{2}+v'\cdot\dfrac{t}{2}}{T}\)
\(\Rightarrow v'=5\left(\dfrac{km}{h}\right)\)
Vậy vận tốc nửa thời gian sau của oto là 5(km/h)
Bài 25:
Ta có: \(v_{tb}=\dfrac{s+s'}{t+t'}=\dfrac{s+s'}{\dfrac{s}{v}+\dfrac{s}{v'}}=\dfrac{s+s'}{\dfrac{1}{12}+\dfrac{1}{v'}}\)
\(\Leftrightarrow8=\dfrac{2}{\dfrac{1}{12}+\dfrac{1}{v'}}\)
Mà: \(8=\dfrac{2}{\dfrac{1}{4}}\)
\(\Rightarrow\dfrac{2}{\dfrac{1}{12}+\dfrac{1}{v'}}=\dfrac{2}{\dfrac{1}{4}}\)
\(\Rightarrow\dfrac{1}{v'}=\dfrac{1}{4}-\dfrac{1}{12}\)
\(\Rightarrow\dfrac{1}{v'}=\dfrac{1}{6}\)\(\Rightarrow v'=6\left(\dfrac{km}{h}\right)\)
