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Qualification => Subject Doubts => GCE AS & A2 Level => Math => Topic started by: nipuna92 on May 10, 2010, 05:23:27 am

Title: P1 Maths October November 2007 question 4 (iii)
Post by: nipuna92 on May 10, 2010, 05:23:27 am

October November 2007 question 4 (iii) ONLY

The 1st term of an arithmetic progression is a and the common difference is d, where d ? 0.

(i) Write down expressions, in terms of a and d, for the 5th term and the 15th term. [1]
The 1st term, the 5th term and the 15th term of the arithmetic progression are the first three terms of
a geometric progression.
(ii) Show that 3a = 8d. [3]
(iii) Find the common ratio of the geometric progression. [2]
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Please include all ur steps and thankx in advance

Title: Re: P1 Maths October November 2007 question 4 (iii)
Post by: nid404 on May 10, 2010, 05:47:28 am

ohk....but i have to do all to get the third one...you should've posted your answer...anyway it's ok

i) a+(n-1)d
  5th term= a+4d
   15term=a+14d

ii) the 3 terms of G.P are

a, a+4d, a+14d,....

a+4d/a= a+14d/a+4d

this gives you
3a=8d

iii) ar/a=r

take the ratio of any 2 terms

eq 1 a+4d/ a   or also a+14d/a+4d

substitute either a or d with the other.

3a=8d
a=8/3 d
sub this for eq 1

(8/3d + 4d)/ (8/3d) = 2.5
r=2.5

Title: Re: P1 Maths October November 2007 question 4 (iii)
Post by: nipuna92 on May 10, 2010, 05:56:29 am

thanx a lot. 8)

Title: Re: P1 Maths October November 2007 question 4 (iii)
Post by: nid404 on May 10, 2010, 05:58:14 am

No problem  :)