A projectile can have same range R for two angles of projection. It t1 and t2 are 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 90∘−θt1=2usin⁡θg,t2=2usin⁡90∘−θg=2ucos⁡θg t1t2=2usin⁡θg×2ucos⁡θg=2gu2sin⁡2θg=2gR  where R=u2sin⁡2θ   Hence, t1t2∝R        (as R isconstant)