When I want to write something longer than a tweet about something other than math or sci-fi, here is where I'll write it.

## Wednesday, September 29, 2010

### Wednesday Math, Vol. 129: The trouble with trigonometry

Here's the thing about trig. With a tiny amount of information, you can figure out a whole passel of stuff. Let's assume that this is the unit circle, which means it all the points where

x² + y² = 1. The x coordinate is the cosine of the angle and the y coordinate is the sine. There are four other trig functions, but once you've got sine and cosine, they can be derived as follows.

tangent = sine/cosine

cotangent = cosine/sine

secant = 1/cosine

cosecant = 1/sine.

If I give you anyone of these values and I tell you what quadrant the angle is in, you can find all the rest of the values. That means two pieces of information gives you five more.

Except, not really. Two pieces of information gives you forty seven more. Here's why.

I've marked eight angles in the picture, a and seven more that have some symmetrical relation to a. If we say the values at angle a are (x, y), then we also know the values at the other seven points.

a °-> (x, y)

(90 - a)°-> (y, x)

(90 + a)°-> (-y, x)

(180 - a)°-> (-x, y)

(180 + a)°-> (-x, -y)

(270 - a)°-> (-y, -x)

(270 + a)°-> (y, -x)

(360 - a)°-> (x, -y)

Confused yet? So are my students. It's a lot of stuff to grasp, but it's that first glimpse into symmetry, the most important concept in mathematics. Just a little information and the assumption of symmetry can unlock so many tricky puzzles if your mind works that way.

I am completely convinced not every mind works that way, and to have a mind that works that way and thinks this is kind of cool... an even smaller set.

The struggle continues.

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## 2 comments:

we werent allowed to look at the trig tables in geometry

which means i actually understand what you are talking about

sort of

When you mentioned the symmetrical relationship between

aand all the other angles, I started mentally folding the circle to see every axis.I liked trig. I especially liked trig when I started getting to use it in calculus. Math has a lot of things that, once you get your head around them, make everything so much easier.

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