Arithmetic sequence

**christina** · January 10th 2010, 04:06 AM

Vivienne starts counting going by 3s from 4:
4,7,10,13….
Chloe starts counting backwards from 744 by 7s:
744, 737, 730, 723
They start saying their sequences at the same time and simultaneously say one number every second
i)What is the difference between the 7th numbers they both say?
(Using the arithmetic sequence formula) – I got 682 which is not the right answer ><
ii)Which number do they both say at the same time?
I need some help with this question, thank you for any help given

**e^(i*pi)** · January 10th 2010, 04:26 AM

Originally Posted by christina

Vivienne starts counting going by 3s from 4:
4,7,10,13….
Chloe starts
counting backwards from 744 by 7s:
744, 737, 730, 723
They start saying their sequences at the same time and simultaneously say one number every second
i)What is the difference between the 7th numbers they both say?
(Using the arithmetic sequence formula) – I got 682 which is not the right answer ><
ii)Which number do they both say at the same time?
I need some help with this question, thank you for any help given

Use the formula for the nth term of an arithmetic sequence $U_n = a+(n-1)d$ . For example

$U_7 = a+6d$

Spoiler:

**Prove It** · January 10th 2010, 04:28 AM

Originally Posted by christina

Vivienne starts counting going by 3s from 4:
4,7,10,13….
Chloe starts counting backwards from 744 by 7s:
744, 737, 730, 723
They start saying their sequences at the same time and simultaneously say one number every second
i)What is the difference between the 7th numbers they both say?
(Using the arithmetic sequence formula) – I got 682 which is not the right answer ><
ii)Which number do they both say at the same time?
I need some help with this question, thank you for any help given

First sequence:

$t_n = 4 + 3(n - 1)$

so $t_7 = 4 + 3(7 - 1)$

$= 22$ .

Second sequence:

$t_n = 744 - 7(n - 1)$

so $t_7 = 744 - 7(7 - 1)$

$= 702$ .

So the difference between them is $702 - 22 = 680$ .

**Prove It** · January 10th 2010, 04:30 AM

Originally Posted by e^(i*pi)

Use the formula for the nth term of an arithmetic sequence $U_n = a+(n-1)d$ . For example

$U_7 = a+6d$

Spoiler:

Actually Viv's is

$U_n = 4 + 3(n - 1)$

$U_7 = 4 + 3(7 - 1)$

$= 22$ .

**e^(i*pi)** · January 10th 2010, 04:45 AM

Originally Posted by Prove It

Actually Viv's is

$U_n = 4 + 3(n - 1)$

$U_7 = 4 + 3(7 - 1)$

$= 22$ .

Yeah it is, for some reason I put the common difference as 4

**HallsofIvy** · January 10th 2010, 04:48 AM

Originally Posted by christina

Vivienne starts counting going by 3s from 4:
4,7,10,13….
Chloe starts counting backwards from 744 by 7s:
744, 737, 730, 723
They start saying their sequences at the same time and simultaneously say one number every second
i)What is the difference between the 7th numbers they both say?
(Using the arithmetic sequence formula) – I got 682 which is not the right answer ><
ii)Which number do they both say at the same time?
I need some help with this question, thank you for any help given

Since they are saying the numbers at intervals of one second, they will say $a_n$ and $b_n$ at second "n". The first sequence, as other have told you is $a_n= 4+ 3(n-1)$ and the other is $b_n= 774- 5(n-1)$ .

To find the number they say both say at the same time, solve $a_n= 4+ 3(n-1)= 774- 5(n-1)= b_n$ for n, then put it into either of the formulas to find the number.

**christina** · January 11th 2010, 01:41 AM

thank you so much for all your help guys ! i really appreciate it ^^

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