Below is a possible solution to Challenge 55 - Make an Alphabetic Triangle

Put following formula AB4 somewhere and drag to the right.

=IFERROR(IF(AND(ROWS($1:1)=1,COLUMNS($AB:AB)=1),"A",IF(ROWS($1:1)>26,"",
IF(AB3<>"",AB3,IF(IF(AA3="",0,1)+IF(AB3="",0,1)+IF(AC3="",0,1)>0,CHAR(ROWS($1:1)-1+65),"")))),"")

(If you are putting the formula somewhere else, adjust references accordingly)

The Excel file containing the above solution can be downloaded from Solution - Challenge 55 - Make an Alphabetic Triangle