# Expanding Sums, Products, Containing Variables?

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### #1 AussieGuy

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Posted 03 April 2013 - 02:25 PM

I have an expression like this:

sum(product(x-i,i,1,n)/n!,n,2,5)

which I'd like to expand to obtain a polynomial in x. But try as I might, all I can see (in the Main screen) is a symbolic representation of the sum or products, I can't get the ClassPad to expand it all out. How can I do this? ("expand" doesn't seem to have any effect here).

Thanks!

### #2 helder7

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Posted 03 April 2013 - 04:20 PM

i think that you can not expand a sum( or product(

### #3 AussieGuy

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Posted 03 April 2013 - 06:57 PM

Really? Why on earth not? That seems like a bizarre restriction. What options are there - are there alternatives to sum & product which are expandable?

### #4 helder7

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Posted 03 April 2013 - 09:31 PM

I I have not really explored (in detail) the cas system that classpad have. The quick tests that i do i could not do it.

There is a Σ in math keys, test it.I dont know if work like sum( .

Edited by helder7, 03 April 2013 - 09:32 PM.

### #5 AussieGuy

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Posted 03 April 2013 - 11:11 PM

Σ and sum are the same. I have discovered a solution though: put the objects to be summed into a sequence:

seq(∏(x-i,i,1,n)/n!,n,1,5) => s

sum(s)

I don't know why Σ doesn't work directly.

### #6 MicroPro

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Posted 17 May 2013 - 07:11 PM

Maybe CP uses some kind of lazy evaluation or something in Σ and Π functions that prevents it from expanding the inner products.

Anyway yes this is a very silly limitation, at least there should be a way to force it to expand the inner product, but I couldn't find any.

Define sum2(e,x,a,b)=sum(seq(e,x,a,b)) and using this sum2 function instead of the original Σ function may help in this situation.

### Also tagged with one or more of these keywords: classpad 330, sum, product, symbolic

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