Below is a possible solution to Challenge 36 – Generate Triangular Numbers.

Enter below formula anywhere and drag down -

=IF(ROWS($1:1)=1,1,INDIRECT(ADDRESS(ROW()-1,COLUMN()))+ROWS(($1:1)))

A workbook containing the above solution can be downloaded from Solution - Challenge 36 – Generate Triangular Numbers.

Edit - 23-Aug-16 - A better solution is to use below formula and drag down

=ROWS($1:1)*(ROWS($1:1)+1)/2