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You now know the meaning of x in $y = \sin(x)$. It is an arc length on a circle measured in units of the circle’s radius. But what is the meaning of y? First, some background.
A chord is a line segment with endpoints on a circle. Early Greeks were interested in chords because they used inscribed polygons (each of the polygon’s sides was a chord to a circle) to approximate a circle’s circumference. They could calculate the perimeter of the polygon, thereby estimating the circumference of the circle.
According to Wikipedia, the word "sine" comes from the Latin word sinus, or curve. Sinus is "a Latin mistranslation of the Arabic jib, which [itself] is a transliteration of the Sanskrit word for half a chord, jya-ardha".
In Figure 2.2.3, BC is a half chord to the circle on which C lies. In Figure 2.5.1, the varying angle’s measure is labeled θ and the length of the half chord that θ determines is labeled sin(θ). As θ varies, sin(θ) varies accordingly. Thus, sin(θ), the length of the half chord determined by θ, is a function of θ.
Notice that if we use the old, triangle definition of sine after embedding the triangle in a circle, the sine of the central angle in Figure 2.10 is opposite/hypotenuse. So, $$\begin{align} \sin(\theta)&=\frac {\text{length of opposite side}}{\text{length of hypotenuse}}\\[1ex] &=\frac{\text{length of opposite side}} {1}\\[1ex] &=\text{length of opposite side}\end{align}$$
So our new way of thinking about sine fits the old way of thinking about it. But the new way of thinking about sine is tightly integrated with measuring angles.
Putting a triangle in a circle gave us, for free, a way to relate the sine of an angle with the measure of it. Even more comes free. The definition of cosine of an angle in a right triangle is $\cos(\theta) = \mathrm{\frac{adjacent}{hypotenuse}}$. In the unit circle, the hypotenuse is 1. So in Figure 2.5.1,
$$\begin{align} \cos(\theta)&=\frac {\text{length of adjacent side}}{\text{length of hypotenuse}}\\[1ex] &=\frac{\text{length of adjacent side}} {1}\\[1ex] &=\text{length of adjacent side}\end{align}$$Figure 2.5.2 embeds the circle into a coordinate system. Angle measures $\theta$ are still arc lengths on the circle, measured in units of the radius.
On the unit circle, $\cos(\theta)$ and $\sin(\theta)$ are the horizontal and vertical coordinates of the point on the circle at which the arc terminates.
Notice that sine and cosine are each measured in units of the circle’s radius. Values of $\sin(x)$ and $\cos(x)$ are therefore each a fraction of the circle’s radius. That is, values of x, $\sin(x)$ and $\cos(x)$ are all in radians.
We now have all the pieces to understand the graphs of $y = \sin(x)$ and $y = \cos(x)$ in a rectangular coordinate system.
Examine the animation in Figure 2.5.2, focusing on how the sine of the arc length varies with the arc length. In the animation, the sine of the arc length:
Figure 2.5.3 displays this covariation of arc length and sine of that arc length. Look back and forth between left and right animations to see how they show the same thing regarding the way arc length and sine of that arc length vary together. Note: The right animation's scale is smaller the left animation's scale so that you can see a complete cycle on the left being graphed on the right.
Now examine Figure 2.5.4. It shows the covariation of arc length and cosine of that arc length. Look back and forth between left and right animations to see how they show the same thing regarding the way arc length and cosine of that arc length vary together. Note: The right animation's scale is smaller the left animation's scale so that you can see a complete cycle on the left being graphed on the right.
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