Math Symbols
Plus-minus, square root, infinity and the not-equal sign all have Windows Alt codes, left over from scientific typing before Unicode existed. Most other mathematical notation, summation, set operators, logical quantifiers, has no Alt code at all, since the DOS-era character set was never built for a full equation.
For those, the Unicode Mathematical Operators block and its neighbours are the only route: copy the symbol directly, use the HTML entity in a web page, or enter the Unicode value through whatever math or LaTeX tool is in use.
Questions
What is the Alt code for the square root sign?
Alt+251, one of the original DOS codes, with no leading zero.
Is there an Alt code for the summation symbol?
No. Use the HTML entity ∑ or the Unicode value U+2211.
What is the difference between the multiplication sign and a lowercase x?
The multiplication sign, Alt+0215, is a distinct Unicode character, U+00D7, built to read clearly as a math operator rather than a letter.
What is the Alt code for the not-equal sign?
There is no standard Windows Alt code for it. Use the HTML entity ≠ or the Unicode value U+2260.
Why does “x” sometimes get used for multiplication instead of the proper sign?
The letter x is faster to type and sits on every keyboard, so it became a common stand-in long before the multiplication sign, ×, was easy to enter.