Open the slide called Glide Angle (with text) to learn about how a glide angle measures the flying efficiency of the glider

If a glider is in a steady (constant velocity and no acceleration) descent, it loses altitude as it travels across the ground. The glider's flight path will be a simple straight line, shown as the inclined red line in the figure. The flight path will intersect the ground at an angle "a" called the glide angle. The top line in the figure represents the horizontal path of the plane before its descent and is parallel to the ground.

Problem: 

Prove angle a is congruent to angle b, given the above relationships.