RESPECT

By pure happenstance, I happened to take this earlier this year in preparation for a promotion.

https://www.khanacademy.org/math/trigonometry

It's learn as you go, at your own pace. If you ever feel stuck, they also have other math lessons so you can get back up to speed.

The other resource I recommend is this YouTube channel. He's got a bunch of great videos, but I like this recent one he did https://www.youtube.com/watch?v=EElZjxhQxHs

Short version: Start at whatever math level you feel you can do confidently, grab that textbook, and work up from there. I highly recommend the Singapore method - there is a reason Singapore leads the world in math scores.

Long version: I actually never grasped math at all in school due to a combination of undiagnosed dyscalculia/bad math teaching in elementary and teachers who would get frustrated and do it for me on tests in middle/high school. I could memorize steps long enough to fake my way through a unit, but I had NO grasp of the actual material. (I did great in geometry because proofs required explaining why you get a result, not just "do these steps.")

So in my 40s, crying over basic conversions on the amateur radio exam, I actually bought myself a set of elementary math textbooks and started from counting and number relationships (pairs of numbers that make 10 for example).

IT CHANGED MY LIFE. And algebra onward was easy because I finally learned to understand the relationships, not just memorize steps.

So yeah. Start with the math you know and look for books/materials that clarify how you get certain results and not just what steps to plug in. Make it interesting to you. Is my two cents.

At the time of writing: 3 different replies, all different, all good. It depends on how your brain works, unfortunately!

For elementary math concepts, middle school books explain stuff in a variety of different ways. So you can shop around for that. I inherited a math book from my father and it was enlightening! It explains concepts I already knew in different ways and suddenly the connections between them and others were so clear to me.

Specifically for sinus, what you are looking for is a section of trigonometry. You are lucky in the sense that it’s a fairly “independent” set of concepts, so you can jump in without too much background. Personally, I would advise getting a college level trigonometry book and read the first chapter. If that doesn’t work, going to high school level and different ways of presenting the topic.

I've personally had success by diversifying the learning process as much as possible. Read the book chapter on the subject you want to learn. Then find a few different YouTube videos on the subject. Then read a different book, or a Wikipedia article, or whatever media you can find. Don't try to understand a complex topic in one shot, from one source. Let the understanding develop slowly and naturally.

Edit: forgot to mention that you should treat math just like learning a musical instrument or a sport, in the sense that regular practice will work wonders. Just try to consistently spend 20-30 minutes every day practicing math.

Read the Wikipedia article on sine and cosine and work backwards from there. At the high school level, the “elementary understanding” section is usually as far as they go. Understanding the math behind electricity is at an electrical engineering/university level and it’s going to be counterproductive to go down that rabbit hole when trying to learn fundamentals.