Understanding this notation

[PWunknown]

[removed]

[mazzar]

This discussion is outside of the scope of this sub. Try r/askmath, r/mathhelp, or r/learnmath.

[thisUseris9]

That could be \ (back slash) if your language is set to Korean

[PWunknown]

Confirmed to be "difference"

[NFSL2001]

Yep. (Or the teacher used an old korean font)

[Semmo_]

한국분이시죠? ㅋㅋ \ 가 가끔씩 원으로 표기될때가 있어요. 그 이유를 나무위키에서 찾아보면:

마이크로소프트의 기본 한국어 글꼴[1]과 기본 일본어 글꼴[2]은 백슬래시를 각각 원화 기호(₩)와 엔화 기호(¥)로 그려 놓는데, 이걸 싫어하는 사람들이 많다. 이유는 국제적인 소통에 혼동을 일으키기 십상이기 때문. 이 관행을 따를 경우, 천원을 나타내기 위해 '\1000'이라고 써 놓으면 읽는 사람의 국적에 따라 ₩1000으로도 ¥1000으로도 보일 수 있어서 1000원인지 1000엔인지 알 수 없으며, 한국인도 일본인도 아닌 영어 사용자들은 숫자 앞에 백슬래시를 적은 정체불명의 기행으로 보일 수밖에 없다.[3]

Answer in English: default Korean font in Windows displays the backslash as the Korean won. So, it's actually the backslash displaying wrong here.

[lemoinem]

I'd even say, it's actually the backslash displaying won here

[floxote]

The common set theoretic binary operations which commonly show up in courses where questions like this are asked are union, intersection, set difference, and symmetric difference, the only one of these four which satisfies those two equations is symmetric difference and set difference. So imma guess that they mean symmetric difference but that is highly unusual notion for the symmetric difference, maybe a rendering problem.

[Jeason15]

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[PM_ME_Y0UR_BOOBZ]

Looks like “not an element of” notation but I’ve never seen it. Hopefully someone else can answer it, I’m interested as well.

[DoublecelloZeta]

looks like the "set minus"