Question

Answer 1

Gọi vận tốc dự định của 2 bố con là a(km/h) \(\left(a>0\right)\)

Theo đề,ta có: \(\dfrac{2}{3}.\dfrac{60}{a}+\dfrac{1}{3}.\dfrac{60}{a-10}=\dfrac{60}{a}+\dfrac{1}{6}\)

\(\Leftrightarrow\dfrac{40}{a}+\dfrac{20}{a-10}=\dfrac{60}{a}+\dfrac{1}{6}\Rightarrow\dfrac{20}{a-10}=\dfrac{20}{a}+\dfrac{1}{6}\)

\(\Leftrightarrow\dfrac{20}{a-10}=\dfrac{120+a}{6a}\Rightarrow120a=\left(120+a\right)\left(a-10\right)\)

\(\Leftrightarrow120a=a^2+110a-1200\Leftrightarrow a^2-10a-1200=0\)

\(\Leftrightarrow\left(a-40\right)\left(a+30\right)=0\) mà \(a>0\Rightarrow a=40\)