7.74.3 Can you answer these questions?

7.74.3.1 Error shown Missing $ inserted. even if it does not make sense

https://tex.stackexchange.com/landing/r/digest?cta=question&id=623770

I have is statement 
R_{AB} $=$ \begin{bmatrix} 
It is showing Missing $ inserted. I have no idea about it is there anyone who can help out please 
[math-mode]
 

asked by Arth https://tex.stackexchange.com/landing/r/digest?cta=user&id=257392 1 vote

7.74.3.2 enumitem: other label style

https://tex.stackexchange.com/landing/r/digest?cta=question&id=623210

I am creating a presentation using beamer. Maybe this is a stupid question, but I haven't found something similar on the web: using the packages "enumitem", how can I see different label ... 
[beamer] [enumitem]
 

asked by John_maddon https://tex.stackexchange.com/landing/r/digest?cta=user&id=239548 1 vote

7.74.3.3 siunitx - Underline significant digits in SI object

https://tex.stackexchange.com/landing/r/digest?cta=question&id=623851

Context 
Hello all, 
I am writing a document making use of the siunitx package for my unit formatting. I use the \SI{...}{...} blocks for ease of writing when formatting my numbers. 
In my document, I ... 
[siunitx] [underline]
 

asked by Kneelawk https://tex.stackexchange.com/landing/r/digest?cta=user&id=257423 1 vote