we have
#ArarrB#
#u=v_1" " v=v_2" let the distance be "s" and accln "a#
using #" "v^2=u^+2as#
#color(blue)(v_2^2=v_1^2+2as)--(1)#
#ArarrC#
#u=v_1" " v=v_c" the distance is "s/2" and accln "a#
using #" "v^2=u^+2as#
#v_c^2=v_1^2+2as/2#
#=>color(blue)(v_c^2=v_1^2+as)--(2)#
#eqn (2) xx2" and rearrange"#
#2v_c^2=2v_1^2+2as#
#color(red)(2v_c^2-2v_1^2=2as)#
substitute into#" "(1)#
#v_2^2=v_1^2+color(red)(2v_c^2-2v_1^2)#
now rearrange for #" "v_c#
#v_2^2=2v_c^2-v_1^2#
#v_c^2=(v_2^2+v_1^2)/2#
#v_c=sqrt((v_2^2+v_1^2)/2)#