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I have studied trigonometry for two years. In my current class, we are being taught trigonometric functions (also known as circular functions) in which a (unit) circle is involved whereas in my previous classes we were taught trigonometric ratios in which a right angled triangle was involved. The author states at the beginning of the chapter that: there are two approaches to trigonometry, one using right angled triangle and another using a unit circle and in this class we will study trigonometry using the (unit) circle approach. He writes that the unit circle approach helps us define the trigonometric functions of real numbers which is required in calculus and from there onwards the chapter continues.

My question: What is the difference between the two approaches to trigonometry? Why were we taught the right angled triangle approach at the starting and not the unit circle approach if the latter is comparatively more useful?

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The two approaches are just that - different ways to get at the same underlying mathematics. That mathematics has several different uses. One is to "solve triangles" - figure out some sides and angles when you know the values of other sides and angles. You start with right triangles but then move on to more general ones. That's where you began, and where most studies of trigonometry begin.

It turns out that those same trigonometric functions are also useful when you want to study periodic phenomena, like the length of the day in the year, or springs bobbing up and down, or old-fashioned phonograph turntables. Then it's the approach using the unit circle that gets you to the useful material faster.

It's also thought that the triangle approach is easier to understand than the unit circle approach, which is perhaps why it comes first in the curriculum.

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Let me add something to Ethan's excellent answer: for the unit circle approach, you have to know some way to say "we've gone $t$ units around the unit circle." That means having a way to measure the length of a curve ... which is typically rigorously defined only when you've done "arclength" in calculus. So there's a little bit of a chicken-and-egg problem here. For angles in the plane (and trig functions associated to them), you can either prove from your axioms that congruent angles have the same "measure" (or assume it, if you're using Hilbert's axioms for plane geometry), or simply assign sine and cosine values to "angles" (however you may have defined those), and then prove that if two angles are congruent, their sines are the same, etc. Most geometry and/or trig books don't bother with this -- they say "we can measure angles with a protractor", and move on. That's probably not a bad idea, but it's not a formal definition of angle measure.

Notice, by the way, that the "right angles" approach and the "circle" approach are, for angles between 0 and 90 degrees, essentially identical. For if you draw a radius from the origin $(0, 0)$ to a point $(x, y)$ of the unit circle with $x, y > 0$, then you can drop a line from $(x, y)$ to the point $(x, 0)$ on the $x$-axis, and then go from there back to the origin. That's a right triangle, with hypotenuse 1 (because it's a unit circle!). The sine is therefore just the length of the vertical segment (which is $y$) divided by the hypotenuse (which is $1$), so the sine is $y$. The cosine is similarly $x$.

The only tricky thing here is that I've defined sine and cosine for the angle $s$ subtended at the origin, but the usual circle definition says "you walk a distance $t$ along the unit circle, counterclockwise starting at $(1,0)$, and get to a point $(x, y)$. We then say that $\cos t = x, \sin t = y$." So the question is "Is the length $t$ of the arc between $(1,0)$ and $(x, y)$ the same as the measure $s$ of the angle subtended at the origin?" The answer's "yes", of course, but it's not a completely trivial statement.

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