Summation of numbers limited to user input

[pgreenway]

I am trying to perform a Riemanns sum calculation on a stress/strain matrix. I have attached the sheet if anyone can help me. Under the calculations tab, N1 for example is supposed to use information from the Q11bar tab to the right of it. There are 256 possible layers of a composite matrix in the Q11bar tab and I have a user input on the User Inputs tab to tell the program how many layers it will use. The equation is as follows:

A11 = summation(k = 1 to N) [Q11]k * (Zk - Zk-1) where Zk-Zk-1 is the difference in height of the layers and N is the number of layers of the composite material.

Is there a way to limit the number of summations to the defined user in put of the number of layers? For example if there are only 25 layers, an equation would only perform 25 calculations using the formula for A11 or if there are 256 layers it would do all 256 layers of the Q11bar calculations.

[pgreenway]

Is there a way to use sumif? I'm trying to piece this together but its not easy.

[pgreenway]

Ok, so I figured out how to do it. I set up columns that have the Q value for that layer and the (Zk-Zk-1) for that layer and had them side by side then for the cell that the riemanns sum value is in I used sumproduct. It was not something I have used before but it is highly valuable.