Arithmetic sequences Watch

Flower_girl_16
Badges: 0
#1
Report Thread starter 9 years ago
#1
3 + 7 + 11 +.... Sum = 820

Find how many terms of the given arithmetic series must be taken to reach the given sum.
0
quote
reply
overclocked
Badges: 12
Rep:
?
#2
Report 9 years ago
#2
Use the formula for the sum of an arithmetic sequence.

This should be in your formula book

Sn= n/2(2a+(n-1)d)
0
quote
reply
Rocious
Badges: 1
Rep:
?
#3
Report 9 years ago
#3
What's the formula for the sum of the first n terms?

What's the first term and the common difference?
0
quote
reply
Champagne Supernova
Badges: 20
#4
Report 9 years ago
#4
Use:

Sn = n/2[2a + (n-1)d]

Where d = common difference
a = first term
and Sn = Sum

Then, aim to find n.


Edit: Still not up to scratch on how to use LaTex. Sorry folks.
quote
reply
Andylol
Badges: 14
Rep:
?
#5
Report 9 years ago
#5
(Original post by Champagne Supernova)
Use:

Sn = n/2[2a + (n-1)d]

Where d = common difference
a = first term
and Sn = Sum

Then, aim to find n.
That. :rolleyes:
0
quote
reply
Flower_girl_16
Badges: 0
#6
Report Thread starter 9 years ago
#6
I get 820 = n + 2n^2
0
quote
reply
Andylol
Badges: 14
Rep:
?
#7
Report 9 years ago
#7
Sounds like a quadratic to me... You know what to do?
0
quote
reply
overclocked
Badges: 12
Rep:
?
#8
Report 9 years ago
#8
(Original post by Flower_girl_16)
I get 820 = n + 2n^2
Work from here:

 

820=n/2 (6 + 5n -5 )



820=n/2 (5n + 1)



1640= n(5n + 1)
0
quote
reply
Champagne Supernova
Badges: 20
#9
Report 9 years ago
#9
(Original post by bob9001)
Work from here:

 

820=n/2 (6 + 5n -5 )



820=n/2 (5n + 1)
Why 5n?
The difference is 4, so d = 4, therefore, it'd b 4n -4, no?
quote
reply
Andylol
Badges: 14
Rep:
?
#10
Report 9 years ago
#10
820 -n  -2n^2 = 0
(20 -n)(41 + 2n) = 0
n = 20
0
quote
reply
Flower_girl_16
Badges: 0
#11
Report Thread starter 9 years ago
#11
So 820 = n/2 (6 + 4n - 4)
820 = n/2 (4n + 2)
1640 = n (4n + 2)
1640 =4n^2 + 2n??

Is that not the same as what i said before??
820 = n + 2n^2

CS do i factorise?
0
quote
reply
Champagne Supernova
Badges: 20
#12
Report 9 years ago
#12
(Original post by Flower_girl_16)
So 820 = n/2 (6 + 4n - 4)
820 = n/2 (4n + 2)
1640 = n (4n + 2)
1640 =4n^2 + 2n??

Is that not the same as what i said before??
820 = n + 2n^2

CS do i factorise?
Yep, that's right.
And, yes factorise, using his above method.
quote
reply
Flower_girl_16
Badges: 0
#13
Report Thread starter 9 years ago
#13
Thank you for your help
0
quote
reply
Champagne Supernova
Badges: 20
#14
Report 9 years ago
#14
No worries. Oh, and welcome to TSR :top:
quote
reply
Andylol
Badges: 14
Rep:
?
#15
Report 9 years ago
#15
Yeah factorise it.

Bob9001 it's 820 = n/2 (6 + 4n - 4)

Argh bloody slow TSR these days
0
quote
reply
overclocked
Badges: 12
Rep:
?
#16
Report 9 years ago
#16
Woops

I can't add :tongue:
0
quote
reply
X

Reply to thread

Attached files
Write a reply...
Reply
new posts
Latest
My Feed

See more of what you like on
The Student Room

You can personalise what you see on TSR. Tell us a little about yourself to get started.

Personalise

University open days

  • Sheffield Hallam University
    City Campus Undergraduate
    Thu, 13 Dec '18
  • University of Buckingham
    Open Evening Undergraduate
    Thu, 13 Dec '18
  • University of Lincoln
    Mini Open Day at the Brayford Campus Undergraduate
    Wed, 19 Dec '18

Do you like exams?

Yes (206)
18.61%
No (673)
60.79%
Not really bothered about them (228)
20.6%

Watched Threads

View All