What is the point of $\sin$ or $\cos$ or $\tan$ of an obtuse angle? Don't we use these functions to find a missing side in a right angle triangle? So why would I use it on an obtuse angle?
I have also done a unit circle.
What’s the point of obtuse angle trigonometry
trigonometry
Best Answer
If all you want to do is think about right triangles then you will never encounter obtuse angles and so do not need to know about the values of the trigonometric functions at such angles. But there are triangles with an obtuse angle that you may want to study.
Moreover, the trigonometric functions appear in many contexts that do not involve triangles. They are crucial in understanding periodic phenomena. There you want to know about $\sin x$ for all possible values of $x$ (whether negative or positive, and as large as you wish). Since you have "done the unit circle" you have started studying this context.