Cells multiplying

If you put in 4th cell (P4) =P1 you can do this:

=SUMPRODUCT(P1:P3;P2:P4)

@zbor, I might be wrong but I don't think that would be scalable for 2 to n terms ?

Perhaps something like:

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The above is for the three term A1:C1 example.

To adjust for increasing numbers of unique terms you would alter A1:C1 accordingly (eg to sum all 6 combinations for 4 unique terms you would reference the 4 values A1:D1)
If needed you could use a Dynamic Named Range to simplify the above and make range references adjust as per values added

I'm sure the math guys above will be able to shorten the above or produce a much simplified (non-array) version