[url=https://nrich.maths.org/1013][size=100]https://nrich.maths.org/1013[/size][/url]
The digit sequence 123456789
Insert plus/minus sign(s) in between the digit sequence yielding an arithmetic expression equal/closest to ...
Reference 1: cut-the-knot
[url=https://www.cut-the-knot.org/do_you_know/digits.shtml]https://www.cut-the-knot.org/do_you_know/digits.shtml[/url]
Reference 2: world of numbers
[url=http://www.worldofnumbers.com/ninedig1.htm]http://www.worldofnumbers.com/ninedig1.htm[/url]
Reference 3: nrich