calculate the sum of the muplication of 2 columns in org table

Question

I have a org table:

| a | b | c |
|---+---+---|
| 1 | 2 | i |
| 3 | 4 | j |
|---+---+---|
| 4 | 6 |   |

I want to multiply columns a and b and sum them up (except last row: 1*2+3*4) and put into the last cell:

| a | b | c |
|---+---+---|
| 1 | 2 | i |
| 3 | 4 | j |
|---+---+---|
| 4 | 6 | 14|

What is the formula to achieve this? I tried #+TBLFM: @>$>=sum($1 * $2) without luck.

My org version: Org mode version 9.1.9 (release_9.1.9-65-g5e4542 @ /Applications/Emacs.app/Contents/Resources/lisp/org/)

Answer 1

 @4$3=(@2$1..@3$1)*(@2$2..@3$2)

or

 @>$>=(@<<$<..@>>$<)*(@<<$<<..@>>$<<)

or

 @>$>=(@I$1..@II$1)*(@I$2..@II$2)

using the horizontal lines to delimit instead of explicit row numbers.

The relevant section of the manual is dense but well-worth reading (and re-reading).

Answer 2

You could use the Orgtbl-Aggregate package available on Melpa

The sums are in a separate table, easy to update

#+name: mytable
| a | b | c |
|---+---+---|
| 1 | 2 | i |
| 3 | 4 | j |

#+BEGIN: aggregate :table "mytable" :cols "vsum(a) vsum(b) vsum(a*b)"
| vsum(a) | vsum(b) | vsum(a*b) |
|---------+---------+-----------|
|       4 |       6 |        14 |
#+END: