Two particles, 1 and 2, move with constant velocities `v_1` and `v_2`. At the initial moment their radius vectors are equal to `r_1` and `r_2`. How must these four vectors be interrelated for the particles to collide?

Answer

`[(r_(2)-r_(1))/(|r_(1)-r_(2)|) = (v_(1)-v_(2))/(|v_(1)-v_(2)|)]`