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Sequence and series

  • Thread starter Natasha1
  • Start date
  • #1
467
7
Right here is my sequence 2, 5, 8, 11, 14, ....

I have been asked to prove that the cube of any number in the sequence is in the sequence.

my answer:

General term: a_n=3n+2

We need to cube a_n and see if it matches a number in the series i.e. (a_n)^3 = 3q+2 for some integer q.

(a_n)^3
=27n^3 + 54n^2 + 36n + 8
=3(9n^3 + 18n^2 + 12n + 2) +2
=3k+2

If this is a member of the series, then 3q+2 = 3k+2 for some integer q.

Solving for q:

q = k which is always in the sequence.

So the cube of any number is in this sequence.

But now I'm asked to show which cube numbers (therefore not in the sequence, I think :confused: ) are not in the sequence and to prove it?

A little confused how to do this one could anyone help please :-)
 
Last edited:

Answers and Replies

  • #2
Fermat
Homework Helper
872
1
Check your expansion of (3n+2)^3

(3n+2)^3 = (3n)^3 + 3.(3n)^2.2 + 3.(3n).2^2 + 2^3
(3n+2)^3 = 27n^3 + 54N^2 + 36n + 8
 
  • #3
467
7
Fermat said:
Check your expansion of (3n+2)^3

(3n+2)^3 = (3n)^3 + 3.(3n)^2.2 + 3.(3n).2^2 + 2^3
(3n+2)^3 = 27n^3 + 54N^2 + 36n + 8
oops thanks ever so much!
 
Last edited:
  • #4
467
7
Can anyone see through this one? :bugeye:
 
Last edited:
  • #5
Fermat
Homework Helper
872
1
Your sequence is a_n = 3n + 2

ergo 3n and 3n+1 are not in the sequence.

Does that help?
 
Last edited:
  • #6
467
7
Fermat said:
Your sequence is a_n = 3n + 2

ergo 3n and 3n+1 are not in the sequence.

Does that help?
well spotted! merci!
 
  • #7
Fermat
Homework Helper
872
1
c'est rien!
 

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