A projectile can have the same range R for two angles of projection. If t1 and t2 be the times of flights in the two cases, then the product of the two time of flights is proportional to
Two projectile have same range if their angle of projections are complementary. Range R=u2sin2θg
t1=2usinθgt1=2usin(90−θ)gt1t2=4u2sinθcosθg2=2u2sin2θg2=2Rg
The product of the two time of flights is proportional to R.