Back
to top
C1 Recursion Sequence help
Watch this threadPage 1 of 1
Go to first unread
Skip to page:
HappyHumanist
Badges:
2
Rep:
?
You'll earn badges for being active around the site. Rep gems come when your posts are rated by other community members.
#1
Hello all, been doing some past papers and I'm a bit confused as too why one of my methods isn't working: it's Question 8 on Edexcel C1 June 2007 (see http://www.examsolutions.net/a-level...June/paper.php).
A sequence a1, a2, a3 is defined.
a1 = k
an+1=3an+5 n>1
Where k is a positive integer
You are then asked to calculate the sum of the first four terms. The correct way I've found out now is too simply add them up one by one (e.g. write out a1, a2, a3 and a4 and then just sum them), but I'm a bit confused. I tried to use the formula: Sn=n/2(a+L), where n=4, a=k and l=27k +65 (a4), but this gives me an incorrect answer of 56k + 130. The correct answer is 40k + 90. Why doesn't using this formula work? Any help appreciated.
A sequence a1, a2, a3 is defined.
a1 = k
an+1=3an+5 n>1
Where k is a positive integer
You are then asked to calculate the sum of the first four terms. The correct way I've found out now is too simply add them up one by one (e.g. write out a1, a2, a3 and a4 and then just sum them), but I'm a bit confused. I tried to use the formula: Sn=n/2(a+L), where n=4, a=k and l=27k +65 (a4), but this gives me an incorrect answer of 56k + 130. The correct answer is 40k + 90. Why doesn't using this formula work? Any help appreciated.
0
reply
m4ths/maths247
Badges:
7
Rep:
?
You'll earn badges for being active around the site. Rep gems come when your posts are rated by other community members.
#2
HappyHumanist
Badges:
2
Rep:
?
You'll earn badges for being active around the site. Rep gems come when your posts are rated by other community members.
#3
(Original post by m4ths/maths247)
First question....is it arithmetic?
First question....is it arithmetic?
The terms are:
a1 = k
a2 = 3k + 5
a3 = 9k + 20
a4 = 27k + 65
Thanks!
0
reply
m4ths/maths247
Badges:
7
Rep:
?
You'll earn badges for being active around the site. Rep gems come when your posts are rated by other community members.
#4
Report
#4
(Original post by HappyHumanist)
I thought all sequences defined by a recursion relationship were!
The terms are:
a1 = k
a2 = 3k + 5
a3 = 9k + 20
a4 = 27k + 65
Thanks!
I thought all sequences defined by a recursion relationship were!
The terms are:
a1 = k
a2 = 3k + 5
a3 = 9k + 20
a4 = 27k + 65
Thanks!
1
reply
HappyHumanist
Badges:
2
Rep:
?
You'll earn badges for being active around the site. Rep gems come when your posts are rated by other community members.
#5
(Original post by m4ths/maths247)
By letting k=1 does that help you understand why the use of the formula was not successful?
By letting k=1 does that help you understand why the use of the formula was not successful?
0
reply
TheScallywag
Badges:
0
Rep:
?
You'll earn badges for being active around the site. Rep gems come when your posts are rated by other community members.
#6
Report
#6
You simply add all the terms together for the first part which gives you 40k + 90 then take a factor of 10 out to show it can be divided by 10. 10(4k + 9)
0
reply
X
Page 1 of 1
Go to first unread
Skip to page:
Quick Reply
