explain how we know that the cosine ratio for an angle with a terminal arm in quadrant 3 is negative

Question

Trigo math word problems grade 11 

Answer 1

Hi Jhd,

If you draw or view the unit circle, which is crucial for trigonometry, you will see that in quadrant 3, the x-axis is negavtive and cosine relates to the x axis as such: x = rcos(theta), where r is the radius of the unit circle. You see from this relationship that cos(theta) has to be negative in quadrant 3.

By the way, here is alink to the unit circle:

https://upload.wikimedia.org/wikipedia/commons/thumb/4/4c/Unit_circle_angles_color.svg/720px-Unit_circle_angles_color.svg.png