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From: deluth on 21 Apr 2010 02:51
Greetings,

I can't seem to find a formula to do series of sums. I would like to
calculate the following series:

For n = 0 to n=B1/B3
sum: (A1+(A2*n))*A3

Thank you for your help.

Deluth
From: Joe User on 21 Apr 2010 03:03
"deluth" <deluth(a)discussions.microsoft.com> wrote:
> I can't seem to find a formula to do series of sums.
> I would like to calculate the following series:
> For n = 0 to n=B1/B3
> sum: (A1+(A2*n))*A3

I responded to your posting in the same "discussion group" 11 hours earlier.

I wrote....

Well, you could write the following UDF. Note that the variable names a1,
a3, etc do not have to match the cell names. I use them just to help you
relate to the above formula. You can call the UDF from Excel with any cells;
for example: =mysum(A1,A3,A6,B1,B3)

However, note that the series sum can be reduced to a single algebraic
formula. So instead of the UDF, you can write:

=A6* (A1*(B3-B1+1) + A3*(B3-B1)*(B3-B1+1)/2)

which can be simplified further to:

=A6 * (B3-B1+1) * (A1+A3*(B3-B1)/2)

UDF....

Option Explicit
Function mysum(a1 As Double, a3 As Double, _
a6 As Double, b1 As Double, b3 As Double) _
As Double
Dim s As Double, n As Double
s = 0
For n = 0 To b3 - b1
s = s + (a1 + (a3 * n)) * a6
Next
mysum = s
End Function
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