I chose to talk about this because it’s the newest concept and therefore I understand it the least, explaining it will help me.

Quadrantal angles are angles whose terminal sides lie along one of the coordinate axes. Usually the radius of the terminal arm is 1, and since the angle will always be along a axis, one coordinate will be 0. This allows us to use trig ratios without necessarily making a triangle.

Let’s use 180°. Since radius is 1, the arm will go to x=-1 and y is 0. Remember that we can rewrite our trig functions as sinθ=y/r, cosθ=x/r, tanθ=y/x.

From here, just plug in the values.