d equals Distance Flown
h equals Change in Height
a equals Glide angle
depiction of airplane in angle of descent: two parallel lines, one above and one below the plane with line tangent to both at left and designated as "h". The line on the bottom designated as "d" and the two angles created by a line drawn through the plane connecting both parallel lines, designated as "a".
From trigonometry: tan times (a) equals h over d