Okay, this might be a tad silly, but I cam across this graph/picture i my physics textbook, and for the life of me I can't figure out how to connect the two angles. Here, have a look: It's not a big part of what the paragraph is about, so in theory I could just skip it, but it's been killing me to not know! Granted, I haven't brushed up on my Trig. for about, what, 3 years now (aside from the basic stuff that I've so far used in First Year Undergrad Physics and Math), so I'm probably missing something. I tried drawing the "triangles" and trying to find out if there are any opposite/alternate equal angles, but... nothing. The book itself doesn't even mention it beyond the graph, it just takes the equality for granted. Any kind of help is appreciated!