The angel made by relative motion of projectile 1 with respect to 2 with X- axis (horizontal) is:
θ=tan−1u12yu12x
θ=tan−1u12xu12y
θ=sin−1u12xu12y
θ=cos−1u12yu12x
projectile 1 initial velocity=u1; velocity at any point =v1=u1xi^+(u1y-gt)j^ projectile 2 initial velocity=u2; velocity at any point =v2=i u2x+j (u2y-gt) relative velocity of projectile 1 with respect to 2 =v1-v2=i [u1x- u2x] +j [(u1y-gt)-(u2y-gt)] =i u12x+j u12y angel made by relative motion of projectile 1 with respect to θ, then tanθ=u12yu12x