The question is too vague, but here is a source:
"Triangles have so many theorems and rules to remember. If the Pythagorean theorem is so fantastic, then why do we have so many other formulas to remember? Have you heard your teacher mention Soh-Coh-Toa and are still wondering why it is so important? Yeah, you are certainly not alone. The truth is learning the big picture first and then practicing on your own is the best option.
To know what formula to use, we need to break it down into a few simple steps.
1. What are the types of Triangles?
We need to classify a triangle first before we can even attempt to solve for the missing parts.
2. Are you solving for the missing sides or the angles?
Special formulas apply to each case.
3. What are some special cases to remember?
You are going to want to be familiar with special right triangles."
- https://go2tutors.com/triangles-tips-tricks/
