I don't know what you're using this for, but trig is almost never a good approach to anything related to gamedev.
See eg this article for more insight into this: https://iquilezles.org/articles/noacos/

Any

(don't know why gamedev.net keeps cutting off my replies)

I was saying: any representation of lines/planes which uses an explicit “slope” parameter is likely to be awful to work with. The book “Geometric Tools for Computer Graphics” might be a useful reference.

Sounds like a neat book, how difficult?

I think it's not too bad, it starts very basic and provides code samples for everything, which is really useful!