Bài 1:
\(s=vt=40\left(\dfrac{30}{60}\right)=20\left(km\right)\)
Bài 2:
\(\left\{{}\begin{matrix}v'=s':t'=45:3=15\left(\dfrac{km}{h}\right)\\v''=s'':t''=\left(72-45\right):1,5=18\left(\dfrac{km}{h}\right)\\v=\dfrac{s'+s''}{t'+t''}=\dfrac{45+\left(72-45\right)}{3+1,5}=16\left(\dfrac{km}{h}\right)\end{matrix}\right.\)
Bài 1:
Đổi: \(30ph=\dfrac{1}{2}h\)
\(S=v.t=40.\dfrac{1}{2}=20\left(km\right)\)
Bài 2:
Đổi: \(1h30ph=\dfrac{3}{2}h\)
\(\left\{{}\begin{matrix}v_1=\dfrac{S_1}{t_1}=\dfrac{45}{3}=15\left(km/h\right)\\v_2=\dfrac{S_2}{t_2}=\dfrac{72-45}{\dfrac{3}{2}}=18\left(km/h\right)\\v_{tb}=\dfrac{S_1+S_2}{t_1+t_2}=\dfrac{72}{3+\dfrac{3}{2}}=16\left(km/h\right)\end{matrix}\right.\)
