If you want to be a purist, I am thinking the OP's choice of thread title is an impossibility, to start with.
By definition, (math) symbols are not text and, as such, you cannot include symbols into text. Period. As mention above, a plain
text editor (Notepad) will not be able to show you symbols and mathematical formulas, etc.
Sure, you can type html and latex in a text editor, but it looks like we all have the feeling that is not exactly what the OP meant.
The way I think of it is like this: text can only consist of the first 128 ASCII characters. If you are seeing something else, you are not looking at raw text...you are looking to some kind of intelligent rendering of underlying code be it rtf, html, latex, etc.
To be sure, text editors, word processors and document viewers are different kind of applications.
Having said that:
- You will not be able include and see in real time math symbols in Notepad. This is a plain editor program.
- I have never used Wordpad, but it looks like it is some minimal word-processing program. If it does not come with its own equation editor, you may be able to paste a "picture" of an equation.
- Current word-processor kind of program MS-Word, LibreOffice Writer, come with their own Equation Editors.
Alternately, if you think it is easier or you need to add an equation to a different kind of document, there are equation editor kind of programs...one was already mentioned above: Mathtype; which apparently it is not free.
MathCast is, though.