Because you are effectively trying to do the same command twice

A1+A2 is summing those two cells,
SUM is also saying to sum them

You get the correct answer because the A1+A2 is resolved before the SUM acts
upon it, but it is unnecessary, and will be inefficient in a large
spreadsheet.

--
HTH

Bob Phillips

"Ted Rogers" wrote in message
...
I attended an Excel training course yesterday and looking through the
handout this morning noticed these lines in the overview:

'.......when to use '=A1+A2' and when to use '=sum(A1:A10)' and why you
should never use '=sum(A1+A2)'. I can't for the life of me remember the
trainer's explanation and can't find the answer anywhere. Could anyone
help
please?

Ted