side-view of glider (or airplane ) with it's nose facing left. Two perpendicular lines are placed at the center of the fuselage directly over the wing. Another line is drawn parallel to the fuselage of the plane and crosses through the center of the two perpendicular lines. An arrow is drawn from the center of all the intersecting lines on top of the parallel line in the direction of the tail of the plane. Another arrow is drawn from the center of the intersecting lines, upwards approximately fifteen degrees to the left of the perpendicular line running upwards and downwards. Angles are created by all of these lines and are marked as "a", for Glide Angle.
An arrow is drawn from the center of the intersecting lines straight downwards and is marked "W", for weight.
The letter "D" is place to the bottom of the plane and represents Drag.

The following equations are included in the image:

L cosine(a) plus D sine (a) equals W

L sine (a) equals D cosine (a)

with a little more math...
L over D equals cosine (a) over sine (a) equals 1 over tangent (a)

For small angles, tangent (a) equals a times (in radians).