Answer:
3. Usinθ
Step-by-step explanation:
The vertical component of velocity of any object reaching maximum height is 0 and the horizontal component of velocity of any object in flight remains constant.
Therefore the horizontal component of velocity is Ucosθ for both objects at any given time.
At maximum height A is having 0 vertical velocity but B which just starting its flight is having Usinθ as its vetical component of velocity.
Velocity of B relative to A = Velocity of B - Velocity of A
As velocity can be resolved to components,( to simplify the sum )
Horizontal component of velocity of B relative to A = Horizontal component of velocity of B - Horizontal component of velocity of A
which is 3. Usinθ