1. Introduction to Trigonometry Right Triangle Trigonometry

2. Introduction Trigonometry is a branch of mathematics that uses triangles to help you solve problems. Trig is useful to surveyors, engineers, navigators, and machinists (and others too.)

3. Labeling Right Triangles

4. The most important skill you need right now is the ability to correctly label the sides of a right triangle. The most important skill you need right now is the ability to correctly label the sides of a right triangle. The names of the sides are: The names of the sides are:  the hypotenuse  the opposite side  the adjacent side

5. Labeling Right Triangles The hypotenuse is easy to locate because it is always found across from the right angle. Here is the right angle... Since this side is across from the right angle, this must be the hypotenuse.

6. Labeling Right Triangles Before you label the other two sides you must have a reference angle selected. It can be either of the two acute angles. In the triangle below, let’s pick angle B as the reference angle. A B C This will be our reference angle...

7. Labeling Right Triangles Remember, angle B is our reference angle. The hypotenuse is side BC because it is across from the right angle. A B (ref. angle) C hypotenuse

8. Labeling Right Triangles Side AC is across from our reference angle B. So it is labeled: opposite. A B (ref. angle) C opposite hypotenuse

9. Labeling Right Triangles The only side unnamed is side AB. This must be the adjacent side. A B (ref. angle) C adjacent hypotenuse opposite Adjacent means beside or next to

10. Labeling Right Triangles Let’s put it all together. Given that angle B is the reference angle, here is how you must label the triangle: A B (ref. angle) C hypotenuse opposite adjacent

11. Labeling Right Triangles Given the same triangle, how would the sides be labeled if angle C were the reference angle? Will there be any difference?

12. Labeling Right Triangles Angle C is now the reference angle. Side BC is still the hypotenuse since it is across from the right angle. A B C (ref. angle) hypotenuse

13. Labeling Right Triangles However, side AB is now the side opposite since it is across from angle C. A B C (ref. angle) opposite hypotenuse

14. Labeling Right Triangles That leaves side AC to be labeled as the adjacent side. A B C (ref. angle) adjacent hypotenuse opposite

15. Labeling Right Triangles Let’s put it all together. Given that angle C is the reference angle, here is how you must label the triangle: A B C (ref. angle) hypotenuse opposite adjacent

16. Labeling Practice Given that angle X is the reference angle, label all three sides of triangle WXY. Do this on your own. Click to see the answers when you are ready. W X Y

17. Labeling Practice How did you do? Click to try another one... W X Y hypotenuse opposite adjacent

18. Labeling Practice Given that angle R is the reference angle, label the triangle’s sides. Click to see the correct answers. R S T

19. Labeling Practice The answers are shown below: R S T hypotenuse opposite adjacent

20. Which side will never be the reference angle? What are the labels? Hypotenuse, opposite, and adjacent The right angle