2-4 할리데이 10판 솔루션 일반물리학

$$ \begin{cases} v_1 &= 35\ut{km/h} \\ v_2 &= 60\ut{km/h} \\ S &= S_1 = S_2 \end{cases}\\ $$ $$ \begin{aligned} \Ans &= \bar v = \frac{\Sigma S}{\Sigma t} = \frac{S_1+S_2}{t_1+t_2} \\ &= \frac{2S}{\frac{S}{v_1}+\frac{S}{v_2}} \\ &= \frac{2}{\frac{1}{v_1}+\frac{1}{v_2}} \\ &= \frac{2v_1v_2}{v_1+v_2} \\ &= \frac{2 \cdot 35\ut{km/h} \cdot 60\ut{km/h} }{35\ut{km/h} +60\ut{km/h} } \\ &= \frac{840}{19}\ut{km/h} \\ &\approx 44.210526315789473684\ut{km/h} \\ &\approx 44\ut{km/h} \end{aligned} $$