# Law of Cosines

## Cosine

There are 3 basic trigonometric functions and they are sine, cosine, and tangent. We’re going to take a look at **cosine**, and cosine can be abbreviated as cos. The cosine function has a period of 360 degrees or 2Pi radians. What we have here is we have an x-axis and a y-axis, and the x-axis represents the angle measure, and then the y-axis represents the cosine of that angle measure.

The cosine of 0 degrees equals 1, and because of that y equals cosine of x. Basically what this is saying, is that when the angle measure is 0 degrees the cosine is going to be 1. That’s where this function starts, it starts at (0, 1), because we’re at 0 on the x-axis, but we’re at 1 on the y-axis.

This continues in kind of some smooth curves and it’s going to cross the x-axis at (Pi divided by 2, 0), because that’s where it is on the x-axis, and then it’s at 0 on the y-axis. Then this lowest point right here is at (Pi, negative 1) because if we follow it over here it’s at negative 1 on the y-axis.

Then it crosses the x-axis again at (3Pi divided by 2), and then, again, because it’s crossing the x-axis is going to be a (0) on the y-axis. Then it’s highest point, or some people call it its **peak**, is at (2Pi, 1). Again, you just follow this up right here, or you can make a point right here at this highest point and follow it down and you see that it’s at 2Pi, and then you just follow it over and you see that it’s at 1 on the y-axis, so that’s how you figure out the coordinates for this function of cosine. Remember that the cosine function has a period of 360 degrees or 2Pi radians, and remember that y equals cosine of x.