A stone projected with a speed u at an angle θ with the horizontal reaches maximum height H1. When it is projected with speed u at an angle 90−θ with the horizontal, it reaches maximum height H2. The relation between the horizontal range R of the projectile, H1 and H2 is
H1 = u2sin2θ2g and
H2 = u2sin2(90−θ)2g = u2cos2θ2g
H1H2 = u2sin2θ2g × u2cos2θ2g = (u2sin2θ)216g2= R216
∴ R = 4 √H1H2