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

We know that;x=(ucos⁡θ)t and y=(usin⁡θ)t−12gt2 Let x2−x1=u1cos⁡θ1−u2cos⁡θ2t=xy2−y1=u1sin⁡θ1t−12gt2−u2sin⁡θ2t+12gt2=u1sin⁡θ1−u2sin⁡θ2t=Y∴YX=u1sin⁡θ1−u2sin⁡θ2tu1cos⁡θ1−u2cos⁡θ2t=u1sin⁡θ1−u2sin⁡θ2u1cos⁡θ1−u2cos⁡θ2= constant, m (say) ∴Y=mXIt is the equation of straight line passing through the origin.