 This video is called Right Triangle Trigonometry. This video is going to introduce you to some very important vocabulary that's going to help you throughout this entire chapter. This video is crucial to understanding and having success in this chapter. So take very good notes and pay close attention. This chapter is all about right triangles. Just about every triangle you are going to encounter in this chapter will have a 90 degree angle in it. One thing you can know, we need to be able to label all the sides of these right triangles. And one thing hopefully you already know is the name of the side opposite the right angle. So in this case opposite the right angle it will be your longest side. It is always called the hypotenuse. So you can label it accordingly. Then what will happen is in your right triangle you're always going to be looking from a perspective of some angle somewhere. It will rarely if ever be the 90 degree angle. In this case we're going to go from the perspective of angle A. And so ask yourself, we've got two more sides to name. They are the legs of your right triangle. Well think about which one would be considered the opposite of angle A? Would it be the horizontal line on top or the vertical line on the left? Well opposite to me would be the horizontal line on top because it's opposite of that angle A if you're going from the perspective of A. Well then on one side left, the one side that's left a vertical line, well what could that be called? It's not the hypotenuse, it's not opposite, but it is next to angle A. It's touching it, we're going to call that the adjacent side. So please remember, you're going to have in a right triangle a side that you label the hypotenuse and that will always be your side opposite the 90 degree angle. And then your other two sides of your triangle, the legs, one will be opposite, one will be adjacent and that will just depend on what perspective you are looking from. So let's try a few things here. We have a picture of a right triangle ABC. We're going to fill in the blanks and it always just depends on what perspective we're looking from. So for problem one it says for angle A. So that means we're going to be looking from the perspective of angle A in our triangle. They want to know what BC is. Well BC is this side here. Well let's think about it. From the perspective of A, I've got my hypotenuse opposite the 90 degree angle. So what would BC become? Is it the one that's next to angle A adjacent to it or is it across or opposite? I would definitely say it's across or opposite. So BC is the opposite. And then AC, that would be this one sitting right here next to it. Well it's not the hypotenuse, so it will be the adjacent one. Two says now they want you to go from the perspective of angle B. So now things change a little bit. My hypotenuse is going to stay my hypotenuse because it's still the side opposite your 90 degree angle. But now it asks about AC. Well AC isn't my adjacent anymore. It's not touching to angle B. Now it's become the opposite one. Or the opposite segment I should say. And then AB, ABO. Well from the perspective of angle B, angle side AB is touching angle B. So it's adjacent. But remember it is even more importantly than being adjacent it's the hypotenuse. Think of the hypotenuse as being something that kind of trumps everything. Even though AB absolutely is adjacent to angle B, which is the perspective we're going from, because it's the side opposite your 90 degree angle it would be the hypotenuse. And if something's a hypotenuse, call it that. Don't call it adjacent. So just think of hypotenuse trumps adjacent or opposite.