Assume I have numbers in cells A1:A5 and B1:B5. I would like to add them in
column C, but following this formula: C1=A1+B5, C2=A2+B2,...,C5=A5+C1. How
do I do it so that I can just use copy paste of this formula without
manually re-entering it in every cell? Thanks.

Sorry for messing you about. I found a solution to my previous question:

B1=SUMPRODUCT($A$1:INDEX($A$1:$A$5,ROWS(A1:$A$5)),A1:$A$5)
B2=SUMPRODUCT($A$1:INDEX($A$1:$A$5,ROWS(A2:$A$5)),A2:$A$5), etc.
B5=SUMPRODUCT($A$1:INDEX($A$1:$A$5,ROWS(A5:$A$5)),A5:$A$5)

Sorry for messing you about. I found a solution to my previous question:

B1=SUMPRODUCT($A$1:INDEX($A$1:$A$5,ROWS(A1:$A$5)),A1:$A$5)
B2=SUMPRODUCT($A$1:INDEX($A$1:$A$5,ROWS(A2:$A$5)),A2:$A$5), etc.
B5=SUMPRODUCT($A$1:INDEX($A$1:$A$5,ROWS(A5:$A$5)),A5:$A$5)

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