Theo dinh luat bao toan dong luong ta co
$\overrightarrow{p}_{B}+\overrightarrow{p}_{D}=0$ ( Do hat A ban dau dung yen )
$<=>m_{B}.v_{B}=m_{D}.v_{D}$
$<=>m_{B}.K_{B}=m_{D}.K_{D}$
$<=>K_{B}=\frac{D}{B}K_{D}$
Lai theo dinh luat bao toan nang luong ta co
$\Delta E= K_{B}+K_{D}$
$(A-B-D).c^2=K_{D}+\frac{D}{B}K_{D}$
$<=>K_{D}=\frac{B(A-B-D).c^2}{B+D}$
Dap an $B$