Refer to figure.
Consider an aircraft in a straight steady descent along a straight flight path inclined at an angle (γ) to the horizontal. γ (gamma) is the symbol used for descent angle. The forces on the aircraft consist of Lift, perpendicular to the flight path; Thrust and Drag, parallel to it; and Weight, parallel to the force of gravity.

Weight is resolved into two components: one opposite Lift (W cos γ) and the other acting in the same direction as Thrust (W sin γ), forwards along the flight path. The requirements for equilibrium are: Drag must equal the sum of Thrust plus the fowards component of Weight; and Lift must equal its opposing component of Weight.

Forces perpendicular to flight path vector: L = W cos γ

Forces parallel to flight path vector: D = T + W sin γ