A projectile is projected at an angle of 45° to the horizontal. The slope of trajectory of the body varies with time t as

Equation of a projectile y= xtanθ-12gx2u2cos2θ dydt=dxdt tanθ-gxu2cos2θdxdt  dxdt  is constant which is = ucosθ.  So as x inreases with time dydt decreases.