The path of one projectile as seen by an observer on another projectile is a/an:

We know that x = (u cos θ)t  and y = (u sin θ)t-12gt2Let x2-x1 = (u1 cosθ1-u2 cosθ2)t = X      y2 - y1 = (u1 sinθ)t - 12gt2-(u2 sin θ2)t+12gt2                   = (u1 sinθ1-u2 sinθ2)t = YY X = (u1sinθ1-u2sinθ2)t(u1cosθ1-u2cosθ2)t        = u1sinθ1-u2sinθ2u1cosθ1-u2cosθ2        =  constant, m (say)Y = mXIt is the equation of a straight line passing through the origin.