$\therefore K . E =h v-h v_0$
$\therefore \frac{ K E _1}{ K E }=\frac{h v_1-h v_0}{h v_2-h v_0}=\frac{1}{2}$
$\therefore \frac{v_1-v_0}{v_2-v_{01}}=\frac{1}{2}$
$\therefore 2 v_1-2 v_{01}=v_2-v_{01}$
$\therefore v_0=2 v_1-v_2$