Trigonometry is a branch of Mathematics that deals explicitly with the angles of a triangle and tries to see the relationship between each element of the triangle which is three sides and three angles. Trigonometry also deals with the relationship of each angle of triangles with circles and is very specifically used in the branch of science and engineering.
The best and the worst part about trigonometry is that one question has multiple answers. One ratio has multiple formulas, and most importantly, knowing where to use each formula can be really tiresome. It requires a lot of problem solving and memorisation of the formulas. And this is because the whole study is relational. We can find a lot of relations among each element of the triangle.
So the question lies, why must a high school student learn trigonometry and trigonometry formulas? The question is a great one, actually. So if we look at our world, our world consists of a lot of shapes. One of the major ones is a circle, and the other one is a triangle. There are also other shapes like curves and, very specifically, waves. We have tons of places that have waves. Waves from the sea, sound or light, or mechanical waves.