#11 (permalink)

s2 is 11 (term 1 is 2 and term 2 is 9 so 2 + 9 = 11) not 9 so your difference is 7 not 5!
2 + 2 + d = 11
d = 7

Problem solved?

#12 (permalink)

o.O

Nooooo.

You know what, this whole question is just confusing anyway, I'm just going to skip it ^_^;;.

#13 (permalink)

I see where the problem is coming from... in your original post you said sn is defined by 7n-5. Maybe that was poorly written but I'm pretty confident that it means (and what I've been assuming in all of my posts):

an = 7n - 5
not
sn = 7n - 5

This is confirmed by the fact that you're saying plugging in 20 does not work - it is simply giving you a(20).

And obviously no question at that level would say here is the formula; plug in one number (20), and you're done so sn is clearly not equal to 7n - 5!


----

Here is one more way of looking at it. Say that it really is saying:

sn = 7n - 5

Therefore s(20) = 135.

We know that term 1 is that same as s1 which means term 1 = 2.
We know that term 2 can be found by taking s2 (9) and subtracting s1 (2) which gives up term 2 = 7.
Likewise, term 3 will be s3 (16) minus s2 (9) = 7.
Term 4 will be s4 (23) minus s3 (16) = 7.

This gives us 2,7,7,7,7,7,7,7,7,7,7,7,7,7,7,7,7,7,7,7

What would be the point of such a series (and it does add up to 135).

#14 (permalink)

Same thing I was thinking originally, but the teacher worked through the first set which involved finding the first term and the common difference between the terms.He specifically stated that the first term is the same as the sum of the first term, and to find the commom difference you'll have to use s2 and a+ a+d= 9 and find for d. He even confirmed 5 to be the answer and for s 20 to be 1030.

I just don't get why plugging in 20 for n doesn't generate this same thing if we made that same assumption for s2. This is why I was so confused by it and wanted to know why that doesn't work. The question doesn't affect my answer at all, I just want to know so I can get a better answer on how it all works.

He doesn't really explain the concepts to us too well, so I was just checking to see if there was something I was missing. And goddamn, its hard to ask him anything because he dodges questions left and right, and leaves immediately after class ~_~.


<----- Frustrated with math

#15 (permalink)

I don't know what to say to that - he is wrong lol.

There are only two valid interpretations that come to mind:

Either you go my way and say an = 7n - 5 or if you go with sn = 7n - 5 all your terms will look like (for example, you find the a20 by taking s20 minus s19 - doing this with all of them will yield these terms):

2,7,7,7,7,7,7,7,7,7,7,7,7,7,7,7,7,7,7,7

So d is 5 at first but becomes 0 throughout and yes this adds to 135 (and satisfies any other sn you can try) which is s(20) in that equation. Therefore the other equation using d would be invalid since there is no common difference while in my method there is a common difference of 7 as the series is (pasted from Excel):

2 9 16 23 30 37 44 51 58 65 72 79 86 93 100 107 114 121 128 135

So it seems there was a problem mix up somewhere and he plugged d = 5 into that equation when d = 5 only once...