A projectile is fixed at an angleθ with the horizontal, (as shown in the figure), condition under which it lands perpendicular on an inclined plane of inclination α is
The time is given as,
v=u+at
0=vcos(θ−α)−(gsinα)t
t=vcos(θ−α)(gsinα)
The time of flight is given as,
T=2vsin(θ−α)gcosα
Since, both times are equal so, it can be written as,
2vsin(θ−α)gcosα=vcos(θ−α)(gsinα)
cot(θ−α)=2tanα