Sum across cases to create one new case. Do this multiple times to create a new variable

[Allen_Scott]

I have three variables, X, Y and Z. 

I want to create a new variable (A) such that case i in A is equal to SUM OVER j [X(i) + (Z(j) * X(j))].

The SUMPRODUCT function can't handle this. How should I proceed? 

[#2]

@Allen_Scott 

With your permission, next time I suggest you provide more information.

You'll get a quicker and more accurate answer, and those who want to help don't have to invest time guessing the problem.

What I recommend: Always inform your plans about the Excel version, operating system, storage medium/hard drive, OneDrive, Sharepoint, etc.).

If possible, add a file (without sensitive data) and use this file to describe your project step by step, or add photos with the appropriate description.

Don't forget that not every Excel user has a clue about every job and don't forget that what you see he can't see.

In this link you will find some more information about it:

Welcome to your Excel discussion space!

Thank you for your understanding and patience

NikolinoDE

I know I don't know anything (Socrates)

[#3]

Assuming:
i, j, k are row numbers
X, Y, Z are your columns

The general formula you are looking for is something like:
=X1+(X2*Z2)

Then again, this looks like a math problem you are trying to do in excel. Looks like Matix or something.
And @Nikolino is right. You need to add context for your problem. It helps to gets answers quickly. Also, what were you doing in excel, the result you are getting, and the result you are looking for.

[#4]

@NikolinoDE  Many thanks for the reply, I will repost with the extra details.

[#5]

@Allen_Scott 

Are you sure about your problem formulation?  Summing X(i) over j looks odd.  For me, the formula

= X*COUNT(X) + SUM(X*Z)

gives a result, but I doubt it is what you want. 

[Note: CSE is required for older versions of Excel.]