Below is a possible solution to the challenge Challenge 54 - Make a Sequence like A B C D E F......

Put below formula as array formula in a cell and drag down

=IFERROR(CHAR(MATCH(ROWS($1:1),(ROW($1:$26)*(ROW($1:$26)+1))/2,0)+64),"")

Note - Array Formula is not entered by pressing ENTER after entering your formula but by pressing CTRL+SHIFT+ENTER. If you are copying and pasting this formula, take F2 after pasting and CTRL+SHIFT+ENTER. This will put { } brackets around the formula which you can see in Formula Bar. If you edit again, you will have to do CTRL+SHIFT+ENTER again. Don't put { } manually.

The Excel file containing the above solution can be downloaded from Solution - Challenge 54 - Make a Sequence like A B C D E F.....