Question

Help please stuck. 1. Write an explicit formula for the sequence; 3,-1,-5,-9,-13,... then find a14

Answer 1

do u see any pattern in that sequence ?

Answer 2

umm im not sure of what it is :/

Answer 3

try and look at the differnces between each term

Answer 4

so -4

Answer 5

ok so now, if there is a difference of -4 between each term, how could you represent that as an eq?

Answer 6

3+(n-1)-4?

Answer 7

without the correct notation that is gibberish^ please use the full notation

Answer 8

an=a1+(n-1) d
an=3+(n-1)-4?

Answer 9

assume n starts at 1, check it. Does it give you 3? at n=2 do you get -1?

Answer 10

sorry if this sounds dumb but would this be the correct way to set it up?

Answer 11

good try, but not quite

Answer 12

so your n is going to start at 1

Answer 13

let me find you an instructional video

Answer 14

https://www.khanacademy.org/math/precalculus/seq_induction/geometric-sequence-series/v/sigma-notation-sum

Answer 15

^Watch that^

Answer 16

Thanks that helped.. would this be correct?

Answer 17

uhm not quite, so here is an example for the summation \(b_n\) up to term 50

Answer 18

\[\huge\Sigma^{51}_{n=1} b_{n-1}\]

Answer 19

help at all?

Answer 20

also maybe read through here

Answer 21

http://openstudy.com/study#/updates/5361d2f5e4b033b553834456

it's how someone else figured out similar

Answer 22

Okay so am i close? \[\sum_{n=1}^{14}3-4n\]

Answer 23

very close

Answer 24

so your first term should be 3, if n=1, do you get 3?

Answer 25

yes. is the number ontop incorrect (14)?

Answer 26

check again, 3-4(1)=?

Answer 27

wait i got it so it would be 1-4(1)

Answer 28

o.O? Not sure how you got that

Answer 29

i was just asking for what that equaled

Answer 30

youre right so n=0

Answer 31

3-4(1)=? <----what is the question mark?

Answer 32

yes, n can indeed =0

Answer 33

I'm sorry i changed the numbers and got mixed up.

Answer 34

because 3-4(1)=-1

Answer 35

if you change that index, what does the top number have to become?

Answer 36

would it just be -12?

Answer 37

it's never negative, nor is it 12 either, how did you get that?

Answer 38

Because I thought I had to minus -1 from the -13.

Answer 39

well, one -13? why?

Answer 40

Because in the other example I saw that it went down one number

Answer 41

, but why negative 13?

Answer 42

Oh because of the last term in the sequence

Answer 43

ok, yea, the number on top is never negative

Answer 44

Okay I see. So would it be 13? or

Answer 45

yes

Answer 46

\[\sum_{n=0}^{13}3-4n\]

Answer 47

thanks so much. youre the best

Answer 48

finally understood this

Answer 49

np happy you get it, remember, you can ALWAYS check your answer just by finding the first few terms

Answer 50

see if they match up, if they do, you're good

Answer 51

I have a question, when it says find a14 what do i have to do?

Answer 52

@FibonacciChick666

Answer 53

can you tell me what term a_1 is?

Answer 54

its the same equation as 3,-1,-5,-9,-13,... then find a14

Answer 55

so a1 is 3

Answer 56

mhmm

Answer 57

and how do you get that in your eq?

Answer 58

it occurs at n=?

Answer 59

0?

Answer 60

mhmm

Answer 61

so then how would i find a14?

Answer 62

if to find a1 you put in n=0, to find a14 you put in n=?

Answer 63

I dont know if im wrong but would it be n=7

Answer 64

how did you get that? no, just follow the logic from above what is the difference between 0 and 1?

Answer 65

1