Question

Algebra II help !

(picture)

What did I do wrong..?

I always thought at the bottom, where n=? you used the number that the sequence started with, which in this case was -5 & then to the right of the E shape went the general term.

Where have I messed up?

Answer 1

if you are going from -5 to inf, the first of sequence would be 25

Answer 2

does that make sense? so the series shouldn't start from -5

Answer 3

It kind of does, I suppose. How do you get that? Just by multiplying -5 by itself? & sorry it took me so long to respond, my internet is not getting along with this site.. grr.

Answer 4

the site is going through some upgrading and maintenance so it's no biggie - now, you should first look at the bounds of summation which is to infinity indicating it's going positively up. so when you have it set from -5 to positive infinity, you are going from -5, -4, -3...

Answer 5

if you put that as n into the equation and see what values you get, it would be more clear for you

Answer 6

so on that note, you should consider changing lower bound of summation with what you have as summation equation

Answer 7

So @msayer3 what your saying is that I should put what is on the right of the E looking thing should go on the bottom? But I thought that was the general term? & I thought the general term went to the side?

Answer 8

& sorry for all the questions..

Answer 9

Umn, I would disagree with msayer. The upper term is not the value of the out put but the value of n. n is the index of the term. So in the example; the index of -10 is 2 and the index of -20 is four. You would want . The E thing is a capital sigma. Look up big-sigma notation on wikipedia for more.

Answer 10

no, what you have in the right side of summation sign looks correct. Just consider changing n = -5 statement which is what's called the lower bound of summation

Answer 11

@lhm that's where I was getting at

Answer 12

sorry i get my terminologies confused

Answer 13

Wait, so what does all that mean?

Answer 14

so simply, your index term for summation is starting at wrong point

Answer 15

& that is my bottom number, right?

Answer 16

Ah. msayer would be right if you were doing something like sum all the values from 1 to 10 (where it would have just 10 above and i=1 below and i on the right).

Answer 17

@Ihm i think we are thinking the exact same thing

Answer 18

I just reread your first statement. I think so too. >.<

Answer 19

Okay, since y'all know are on the same page now, Lol, what did I do wrong...?

Answer 20

As msayer said, your index term is off. Try expanding the sum a bit like this:\[-5*1+-5*2+-5*3...\] where 1,2,3,etc are values of n. Remember that n increases by 1 each step.
Here's the layout:
\[\sum_{starting-index}^{ending-index} whatever*index\]
Try playing around with the starting index. There rest of what you have is fine.

Answer 21

Okay, but what should I try? Like there has to be a method to the madness.

Answer 22

@msayer3 @Ihm Not to bug y'all, but I don't know what to try

Answer 23

Wait. are you summing the terms in the list or just producing the list?

Answer 24

Just producing the list? I'm honestly not sure. I don't have to add anything so I assume make a list.. Does that make sense/answer your question..?

Answer 25

Ok. (I find the question oddly worded). Either way you can look at it like this:
make some of the series and write the indexes underneath each term.
-5,-10,-15,-20,-25 | term
| index
take the bit to the right of the sigma and plug in the starting index (below the sigma) for n. If it doesn't match the first term, then it's wrong.
Just plug in things like -5, 0, 10, and the like. That will show you how changing the starting index will screw with your sum.

Answer 26

If you fill in the index part of the table above, the first index is your starting index.

Answer 27

So by doing this I find out what goes underneath the stigma?

Answer 28

Yes and why it's important.

Answer 29

I'm sorry, I'm still kind of confused... How will plugging in the numbers give me what goes below? Like I thought it was always what you started with, like your starting number. & Thanks for continuing to help me, I really appreciate it. Like you have no idea.

Answer 30

You needn't plug stuff in to find the right answer- it's just something I thought might help you grasp what it is we're doing when we change the starting index.
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Think of the index (below the sigma) as an address. Each element in the series has its own, unique address. The starting index can be any number but it always points to the first element in the series, the next element will have an address that is one more than the starting index, and so on.

Answer 31

The important thing is the starting index isn't the first number in the series, it's the /address/ of the first number.