Sumar cruzado/crossed summation in excel. Automatic way possible

Hello!

I have a doubt, although is not exactly about what is being discussed here... on summations in excel.
I want to add the values of columns and rows,

A B C
A X 1 3
B 2 X 8
C 4 2 X

To obtain: (A+B and B+A in their respective cells, etc)

A B C
A X 3 7
B 3 X 10
C 7 10 X

I have been checking how but haven't found yet. The problem is that I have huge matrices, that contain thousands of values, so I can't really do by hand one by one, plus I can make errors continously due to mistaking lines...It would take me weeks...

Could anyone tell me how to get this in an automatic way?

Thank you very much!


Best reagards,


miguel