The problem is regarding two projectiles whose distance of separation is known.Their initial velocities and angle of projection are known, plus these angles are complementary and velocities are known to be equal.It is also known that the two projectiles do no colllide. We are required to find the minimum distance b/w the projectiles. I wrote down the displacement vectors for the two(with origin at one of the points of projection,of course).Then I subtracted them and found out the magnitude of the vector.Since this is the distance b/w them I differentiated this w.r.t time(as it is the only variable here) and put that equal to zero to minimize it.Then I put the t obtained back in the eqn. for minimum distance. Is this right? Because the book does not think so!