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# AEA summing a series (sigma notation) watch

1. http://www.mathsexams.ukteachers.com...l/AEA_2004.pdf , Q2.

I just had a go at this question, and don't understand why I'm getting it wrong. It seems to be to do with using a = x as the first term of the geometric series, when I use 1 it works, but a) I don't see why it should be 1, and b) If I use 1, this doesn't work in the next part, the next part only seems to work if I use x as the first term. Could someone explain what I'm supposed to be doing please?
Attached Files
2. Doc3.doc (36.0 KB, 182 views)
3. (b) The sum is: x+2x^2+3x^3+.....=x[1+2x+3x^2+....]=x[1/(1-x)^2)]=x/(1-x)^2
4. Ummm, all you have to do is note that since the n'th term of is , the nth term of is so . Note the change of limits for the sum.

Too slow
5. Ohhh of course. I always do this - make things more complicated than they need be, and then end up getting it wrong cos of it. Thanks to both of you.
You said sum from 1 to infinity (n+1)x^n = 1/(1-x)²
Should be sum from 0 to infinity (n+1)x^n = 1/(1-x)²

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