a, Ta có: $S_{AB}=v_{1}.t+\frac{1}{2}.a.t²$
⇔ $20=2.v_{1}+2a$
Ta có: $v_{2}=v_{1}+a.t$
⇔ $12=v_{1}+2a$
Ta có hpt: $20=2.v_{1}+2a$
và $12=v_{1}+2a$
⇔ $v_{1}=8m/s$
và $a=2m/s²$
b, Ta có: $v_{1}²-v_{0}²=2aS_{OA}$
⇔ $8²-0²=4S_{OA}$
⇔ $S_{OA}=16m$
c, $v_{AB}=\frac{S_{AB}}{t}=\frac{20}{2}=10m/s$
Ta có: $v_{1}=a.t_{1}$
⇔ $8=2.t_{1}$
⇔ $t_{1}=4s$
$v_{OA}=\frac{S_{OA}}{t_{1}}=\frac{16}{4}=4m/s$
$v_{OB}=\frac{S_{OA}+S_{AB}}{t_{1}+t}=\frac{16+20}{4+2}=6m/s$