This paper attempts at establismng a set of flat-trajectory formulas inhe - rent to boundary value problems for ballistic differential equations The propo?sed formulas are simple，clear and precise. Characteristic in the disposition is the diversity of methods utilized and in the forms of representation for the prin?ciple involved. After defining the flat-trajectory is defined and mathematical?ly described the formulas are derived from its differential equations. Based on a numerical solution of the equations principles governing their conduct are then explored.