It isn't. The horizontal and vertical velocities are separate. You separate that velocity into the horizontal component which is constant, disregarding air resistance, and vertical velocity which is changed by the acceleration due to gravity . Make calculations separately because they do not influence each other.
The horizontal component of a projectile's velocity is independent of the vertical component. While the vertical component changes due to gravity, the horizontal component remains constant. Therefore, calculations for these components should be done separately as they do not influence each other.
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