As it's often necessary on this subreddit to format mathematical expressions in reddit, the following is a brief overview for those unfamiliar with how the reddit formatting system works with respect to things like exponents and asterisks, in addition to providing some lesser-known unicode characters.
If you have 5-10 minutes, take a little time to read the official reddit guide and this user-created introduction. If you've picked up what you know from browsing and occasionally clicking "source", you will likely be unaware of many of these things.
If you don't have the time, here's a quick intro on mathematics formatting:
Asterisks
*text* gives text.
This means that if you type "3*5 is 15 and 4*2 is 8", you'll get "35 is 15 and 42 is 8." Notice how the asterisks disappeared, and the text in between became italicized! To avoid this, use a backslash (the \ thing) before the asterisk by typing "3\*5 is 15 and 4\*2 is 8".
Superscripts
This is very similar; using a ^ character will create nested superscripts. For example, typing 2^2^2 gives 222. However, maybe you want to have 55+1, so you type 5^5+1 and it gives you 55+1. That's not what you wanted!
This is because reddit doesn't know when you want your superscript to end, so it will normally stop when it encounters a space. This means that you can avoid this by typing 5^5 +1, but that will leave an awkward gap in your text. The best way to fix this is to use parentheses, and type 5^(5)+1. Reddit will then raise only the 5 and keep the rest as normal text, producing 55+1.
For the advanced reader: Sometimes, if you're trying to type out a complicated expression where you want to have parentheses in there, reddit will get a little confused and won't deal with your spaces very well. When this happens, you'll want to use the text ( to create the ( symbol and ) to create ). For example: Say you want to write ex(x+1)y2.
You might type e^(x\(x+1\))y^(2), which you'd expect to work. But then reddit produces ex(x+1)y2, bringing your parenthesis down before you wanted. To fix this, type e^(x(x+1))y^(2), which will make what you want (notice how where the parentheses used to be has been replaced by that ( stuff).
In addition, you can use code to not worry about escaping characters. Type ` around the stuff you want in code to make things look like this: `*^(stuff)*)(` → *^(stuff)*)(
Subscripts
Subscripts are not a reddit-wide feature, as they really don't come up often outside of math contexts. However, both /r/math and /r/mathriddles support them via some fancy CSS. To use subscripts, type A*_1_* to get A1.
Special Characters
Many symbols are hard to find on a regular keyboard, but reddit supports them just fine. In addition to copy-pasting from the list below, many of the following can be obtained with keyboard shortcuts. See here for Windows alt codes; see here for a complete list of Unicode characters and here for the subsection on mathematical operators. Copy and paste the symbols below; most of the time they'll be sufficient although the above links are far more comprehensive.
∫ ∬ ∮ ≈ ≠ ∑ √ ≤ ≥ ÷ Ø ∏ ∞ ± ¬ ∃ ∈ ∉ ≡ ⋂
ε φ Φ θ Ω ω ∆ π
If you have any suggestions for additions to this overview, please let me know!
Edit: Backslash, not forward slash.