a projectile is fired with velocity u making an angle θ with horizontal, what is the change in velocity when it is at highest point.

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How to find change in velocity

Think about a curve or arc. What happens to a car going uphill if you don't give it more gas? What happens when the car goes downhill? What happened and what caused it?

Think of the motion of the projectile separated into horizontal and vertical components. At the peak of the arc what is its vertical velocity? Does its horizontal velocity change during flight? Then compare the total velocity at its peak with its initial total velocity.