You can find the distance, say across a river, by using similar right triangles. Choosing
a spot on the shore, walk some distance (AE). Here measure your angle to the point
directly across from A. Return to A and walk perpendicular to the river some distance
(DE). Marking this spot, you return to A and walk in the opposite direction until the
angle between the river and the line to the point you marked is the same as the
previous angle. Now you have two similar triangles.
What is the distance across this river?
