show that the angle of projection (theta) for a projectile launched from the origin is given by theta= tan^-1 ( 4 Hmax / R)

 In calculus, a model for projectile motion with no friction is considered, and a "parabolic trajectory" is obtained. If the initial velocity is  and  is the initial angle to the horizontal, then the parametric equations for the horizontal and vertical components of the position vector are
 
(1) , 
 and
(2) . 

Solve equation (1) for t and get , then replace this value of t in equation (2) and the result is

 ,

then try solving, u will get it!