[tex]s_{AB}=4.10-\frac{1}{2}a.10^2=40+50a[/tex]
[tex]s_{BC}=s_{AC}-s_{AB}=4.20+\frac{1}{2}a.20^2-(40+50a)=40+150a[/tex]
[tex]s_{AB}-s_{BC}=-100a=5\Rightarrow a=-0,05(m/s^2)[/tex]
[tex]0=4-0,05t\Rightarrow t=80(s)[/tex]
[tex]-v_0^2=2as\Leftrightarrow -4^2=2.(-0,5).s\Rightarrow s=8(m)[/tex]
Mình sửa rồi bạn nhé :> gõ thiếu