Searching for a specific formula is often rather difficult. A tool which is very suitable for this is the Approach0 search engine.
Some basic information can be found in Guide for New Users (which contains also animations of some examples). If you want to know more about the project, you can look at the documentation.
Approach0 uses mathquill ...
Lists of questions
There are also various lists of questions which might help when searching for some topic (some of them generated by the Stack Exchange software, some of them created manually).
If you click on ask question and fill at least the title (perhaps also tags) the software generates a list of questions that are similar. As pointed out by some ...
You can simply use Google with the restriction to this domain, using (in addition to the search query) site:math.stackexchange.com. (Of course, you can also use any other search engine where this restriction is possible.) For example, you can use the search: triangular numbers site:math.stackexchange.com to get to relevant posts about triangular ...
The site has built-in search engine - the search bar is near the top of the site. Notice that after you get to search results a link with "Advanced Search Tips" become available - it contains various search modifiers. (Advanced search options can also be found in the help center.)
Some useful advice:
Notice that search results can be ...
"Two Children Puzzle / Boy Born on a Tuesday" and variants:
In a family with two children, what are the chances, if one of the children is a girl, that both children are girls?
New Two Children problem
Boy Born on a Tuesday - is it just a language trick?
A "liar paradox" variant: Multiple-choice question about the probability of a ...
I am adding this answer because the question came up again recently and, while much of what I wrote is a little redundant, there is an aspect of the culture of MSE which I think is worth discussing.
In general, a vote on the main site indicates that a post is poorly researched or of low quality. On Meta, votes should, for the most part, be interpreted ...