Excel 365 SUM dynamic array at row level

[L z.]

Hi

(no LAMDBA here). I've been struggling with this for a moment, searched here & there for something similar to no avail.
In the below pic. Dummy arr is the representation of an array that doesn't exist on a sheet, it's calculated in a LET expression and in reality can involve more columns and rows

Question: is there a way to i.e. SUM that array row by row and get the below Expected arr (not on a sheet)?

Thanks
(I posted the question yesterday at a time where the site had issues. Can't find that post today...)

[#2]

Hi @Peter Bartholomew 

I appreciate your follow up on this issue. Since I have the 1st wave of LAMBDAs I figured that one out but this can help others so thanks for sharing it

[#3]

I wasn't trying to patronise! I saw the old discussion on your profile page and though that bringing it up to date might help someone in future if they search the topic.