A projectile goes farthest from the earth when the angle an projection is:

In vertical direction,ui=u sinθ, uf=0 uf2−ui2=−2gs0−u2sin2⁡θ=−2gss=u2sin2⁡θ2gwhere s is the maximum height.For s to be maximum, sinθ should be maximum, i. e., θ = 90°.