For a given velocity, a projectile has the same range R for two angles of projection if t1 and t2 arc the times of flight in the two cases then

For same range angle of projection should be θ and, 90-θ So, time of flights t1=2usin⁡θg and t2=2usin⁡(90−θ)g=2ucos⁡θg By multiplying t1t2=4u2sin⁡θcos⁡θg2t1t2=2gu2sin⁡2θg=2Rg⇒t1t2∝R