Right Triangle
Before we begin learning about the six trigonometric ratios, we need to identify the parts of a right triangle. The following is the right triangle ABC. We will be looking at the triangle from the vantage point of angle A.
The side AB is adjacent to angle A. The side BC is opposite of angle A. The side AC across from the right angle is the longest side and is always called the hypotenuse.
Notice how this changes if we are looking from the vantage point of angle C.
Now side AB is opposite of angle C. Side BC is adjacent to angle C. The hypotenuse remains side AC.
It is very important when working with right triangles that you pay close attention to the angle you are working with. The above diagrams show that adjacent and opposite sides change when the reference angle changes.