# Formula for the nth term

1. Jul 28, 2011

### Nelo

1. The problem statement, all variables and given/known data

Find a formula for the nth term that determines each sequence.

2. Relevant equations

3. The attempt at a solution

c) 6, 5, 4, 3
I said this one was n-1 , but apparently its 7-n . can't it be both? How the heck do you tell which one it is?

f) -3, -6, -12, -24

I said this one was -2n

But somehow its -3(2)^n , i dont get how its not -2n, its just being multiplied by 2 .. Anyone?

2. Jul 28, 2011

### Ivan92

I am going to go off of my memory here. Remember the formula for finding a arithmetic sequence is:

an=a1+(n-1)d

where a1 is the first term of the sequence, an is the nth term of the sequence, and d is the difference between a1 and a2. Whatever the above formula gives you, it should be the answer. It does look like 7-n is the right answer.

So for the ratio one, the formula is:

an=a1rn-1

Same definitions as above except r is the ratio between a1 and a2

Last edited: Jul 28, 2011
3. Jul 28, 2011

### Nelo

So, everything esentially has to be in terms of (n-1) ? Genreally speaking, in terms of the subt/addition

4. Jul 28, 2011

### Ivan92

[STRIKE]Pretty much[/STRIKE]Not always. We can see that a1 = 6, d=-1, so the formula is an=6+(n-1)*-1
you can do some algebraic manipulations to get 7-n.

Edit: I take that back. Not everything will be in n+# terms.

Last edited: Jul 28, 2011
5. Jul 28, 2011

### Nelo

but, if you have something like...

5 , 10 , 15 , 20 ..

you can't say that this is (n+5)

but you can say that its 5n ?

Because 5(n) ?
Where we are now ignoring the 1 completely?

X:

6. Jul 28, 2011

### Ivan92

You are right, we can't say it is n+5. Using your first example, we can see that -1 is the difference between all the terms. Let's put your 5,10,15,20 example into my arithmetic sequence formula. We can see that the difference between all terms is 5 and the first term is 5. So using that information we come up with

an=5+(n-1)*5

an=5+5n-5 (distribution)

an=5n (cancellation of 5-5)

I see what you mean by having it in n+5 terms. You are right it won't always be like n+5, but in your original example, it would be in that form.

7. Jul 28, 2011

### ArcanaNoir

When determining a formula for a sequence, keep in mind that n=1 is the first term unless otherwise specified.

Now, for {6, 5, 4, 3...} you did observe that each term is smaller than the next by 1. That's a good observation. You suggest "n-1". The best thing to do here is see what n-1 actually gives.

Beginning with n=1:
1-1 = 0
2-1 = 1

We may as well end that experiment now, seeing that the first two terms of our sequence don't match the given one. However, the correct answer, "7-n", yields:

7-1 = 6
7-2 = 5
7-3 = 4
7-4 = 3

Which is exactly what we wanted.

Your suggestion n-1 may be intuitively not that far off.
If you consider a recursive sequence (a sequence in which you take the previous term and modify it by something each time to get the next term), you could say:
$a_n=6$

$a_{n+1}=a_n-1$

Can you say now (forget all the recursive talk) what sequence you get if indeed it is defined as "-2n"?

8. Jul 28, 2011

### Nelo

Ok, I understand it now.

here is a word problem.

13) A single bacterium divides into two bacteria every 10 minutes. If the same rate of division continue for 2 hours how many bacteria will there be?

I dont get how to set it up.

2 hours is 120 minutes, the first sequence kinda goes like 1, 2, 4, 8 , 16

Going by 2n.

Its a geometric sequence because its a ratio (division/multi) , but I really dont understand what i am supposed to do. Im supposed to find how many after 120 minutes, but does that mean my sequence goes 10, 20, 30, 40 ?

And my n is 120?

ie) a = 1
r= 2
n= 120
tn cant be 120 cause that would be # of bacteria.. anyone?

9. Jul 28, 2011

### Nelo

Anyone?

10. Jul 28, 2011

### Bohrok

Your sequence will be an = a1r where a1 is the first term and r is the common ratio. Do you know how to get the common ratio?

Last edited: Jul 28, 2011
11. Jul 28, 2011

### Nelo

ratio is 2

12. Jul 28, 2011

### ArcanaNoir

If your bacteria divide every ten minutes, how many times will they divide in two hours? That will be the n.

You have a and r correct. Just remember geometric sequences and series take the form
$ar^{n-1}$

Check it, starting with n=1 and so on, and if it's good, cut right to the desired n.

13. Jul 28, 2011

### ArcanaNoir

You are not going by 2n. That would look like {2(1), 2(2), 2(3),...} which is {2, 4, 6, ...}

You are going by $2a_n$ (each new term multiplied by 2), which is a recursive sequence.

14. Jul 28, 2011

### Nelo

Solve the following

Motion ofa pendulum on the first swing is 50 metres. The length of the arc is 0.97 the previous length on each successive swing. What is the length of the ac after 10 swings?

what about that one? .. makes 0 sense..

assuming the ratio is 0.97, initial sequence is 50, next sequence is 50-0.97, etc

n is 10..

plugged it in as 50(0.97)^10

15. Jul 28, 2011

### ArcanaNoir

The second swing is not 50-(.97). it is 50(.97), but you seem to see that by your attempt at the form of the sequence.

And I'm serious about the form of a geometric sequence. You must put n-1 as the exponent. Try that, check it. Always check the first term, and one or two subsequent terms, and the last term.

Your sequence doesn't even return 50 as the first term.

16. Jul 28, 2011

### Nelo

that gives me 36.8, Which si stilll wrong.

should be 38.01 .

17. Jul 28, 2011

### Nelo

I see

Last edited: Jul 28, 2011
18. Jul 28, 2011

### ArcanaNoir

Try again. Make sure you use n-1. So, what should your exponent be when n=10?

19. Jul 28, 2011

### ArcanaNoir

The new one is not geometric, you even said it, it's arithmetic. That means add or subtract, not multiply.

Last edited: Jul 28, 2011
20. Jul 28, 2011

### ArcanaNoir

Okay so, the rows. Something plus one more each time. N gives the one more, because it goes 1 2 3 etc. So, how do u start with 30? 30+n? Hmm... What do you think?