But why?

Trigonometry is one of those topics that if you get the basics, the rest of it will make a heck of a lot of sense. But if you miss it, you’re going to be trying frantically to come up with ad hoc ways to understand each new concept.[1]

I have been teaching the beginning of trig, and I’ve noticed a few things that I have to watch out for next year:

  1. Have a really good reason prepared for explaining why we care about angles greater than 360. In general, the kids don’t have a good idea of why we’re doing what we’re doing. (“Why do we want to find csc(421)?”; “Why don’t we just say we have an angle of 1 instead of 361? When would we ever need 361 degrees if a circle has only 360?”)[My explanation didn’t hold over well, but it’s true. I said that often times we use angles to measure time, like in a clock. So if we have something repeating — like a spring with a mass in physics, or a ferris wheel going around and around, or a bike wheel spinning — we will be able to model how far it’s gone or how many oscillations its made by using this angle.]
  2. In fact, have a good reason to explain why we care about angles greater than 90. I started out teaching triangles and SOH CAH TOA, and they got it. Then I started teaching how to see angles on the coordinate plane, and I lost some of them. They can — I hope — calculate the sine of 210. But they don’t get why the sine of 210 is at all related to the sine of 30. They see the 210 angle, and they say “where’s our triangle”? And I show them the 30 angle, and they understand that we can form a right triangle with it, but they don’t get why we use the triangle with 30 to deal with the 210 angle.[My explanation dealt with looking at triangles made in the first quadrant, like the sine of 45. I showed them that the opposite side of the triangle was the y-coordinate, the adjacent side with the x-coordinate, and that the hypotenuse was the radius. Then I said the problem with the whole “opposite,” “adjacent,” “hypotenuse,” method of things was that it restricted our angles to lie between 0 and 90. So to expand the domain of these trig functions, we put them on the coordinate plane, and defined sine to be y/r, cosine to be x/r, and tangent to be y/x.But then they asked: “Why? Who cares about angles greater than 90?” Which then takes us back to #1.]
  3. I haven’t given them a big picture, which is part of the problem. Right now they’re learning smaller skills, but they don’t know what the whole point of it is. So what if you can find cot(260)? Why do we care? What does cotangent mean in the real world?

Next year… I might want to motivate trigonometry on the first day, and give a hard application problem that we’ll be able to solve by the end of the unit. Then I’ll give an schematic diagram of what we’ll be doing and try to motivate each step. That will give them the big picture. And hopefully the rest will fall into place once we have the big picture.

[1] It’s like that old adage… “I must’ve been absent that day.” In this case, it’s pretty disastrous.

UPDATE: A student asked me today, “Mr. Shah, will you promise me something?” “Not without knowing what it is.” “Well, will you promise me that you’ll explain why we’re doing all of this at some point?” So I was right on the ball, in terms of students just not knowing what’s going on because they can’t see the forest for the trees. And I went looking in the book to see what applications they have, and they’re awful (kites, ferris wheels, and bicycles). I’ve got to get on this right away!

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