HTML UTF encoding entities

I was doing some project require math symbol. The key was not exist in keyboard
The website that I use as reference is

http://www.fileformat.info/info/unicode/char/b1/index.htm

The link is a plus minus symbol (±). You can search fur further symbol at the top left

Or also here
http://www.danshort.com/HTMLentities/index.php?w=hertz