Proof that Slopes of Perpendicular Lines are Opposite Reciprocals

In this video, I prove that slopes of perpendicular lines are opposite reciprocals of each other. The concepts used in this video include perpendicular lines, slopes of perpendicular lines, altitude rule, similar triangles, proportions in similar triangles, basic algebra, angle sum of triangles, right triangles, geometry.
The big idea in this video is to look for connections between different math concepts.