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# Summation Question Help! watch

1. Attachment 616772I don't know how to do question 7 of this exercise. Showing that the two expressions are equal is fairly straightforward, however I have attempted the second part many times and have failed to get the correct answer. I really don't understand what the right thing to do is. Thanks for any help
2. (Original post by 11owolea)
Attachment 616772I don't know how to do question 7 of this exercise. Showing that the two expressions are equal is fairly straightforward, however I have attempted the second part many times and have failed to get the correct answer. I really don't understand what the right thing to do is. Thanks for any help
Let .
Sum over the expression on both sides over .
Note that the LHS is a difference of partial sums. That is to say:
.

The RHS is then the term that you want, plus a geometric series which you should be able to evaluate.
3. (Original post by joostan)
Let .
Sum over the expression on both sides over .
Note that the LHS is a difference of partial sums. That is to say:
.

The RHS is then the term that you want, plus a geometric series which you should be able to evaluate.
Thanks a lot, is this correct?
4. (Original post by 11owolea)
Thanks a lot, is this correct?
Looks good to me, also for future reference, you can check something like that on Wolfram Alpha
5. I can't see the question, but if you're asking about the sum of ra^(r-1) consider the derivative of the geometric series is also a nice way.

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