A projectile can have the same range R for two angles of projection. If t1 and t2 be the times of flight in the two cases, then what is the product of two times of flight?

The horizontal range is the same for the angles of projection θ and90∘−θt1=2usin⁡θgt2=2usin⁡90∘−θg=2ucos⁡θgt1t2=2usin⁡θg×2ucos⁡θg=2gu2sin⁡2θg=2gR where  R=u2sin⁡2θg Hence,  t1t2∝R.