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Solution of Past Paper of CAIE Math 9709/13 Oct/Nov 2019 Question # 9 $2.99   Add to cart

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Solution of Past Paper of CAIE Math 9709/13 Oct/Nov 2019 Question # 9

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This document presents most carefully worked out and thoroughly explained complete Solution of Past Paper of Cambridge AS/A Level Mathematics (9709), Pure Mathematics 1. We also provide on-on-one online help for students who still need some help beyond our provided solution. We always openly, an...

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  • October 21, 2023
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CIE/GCE/AS/Math/P1/19/Nov/13/Q#9

Question


The first, second and third terms of a geometric progression are 3𝑘, 5𝑘 − 6 and 6𝑘 − 4
respectively.
(i) Show that 𝑘 satisfies the equation 7k2 − 48k + 36 = 0.
(i) Find, showing all necessary working, the exact values of the common
ratio corresponding to each of the possible values of k.
(ii) One of these ratios gives a progression which is convergent. Find the
sum to infinity.


Solution


i.


From the given information, we can collect following information about this Geometric
Progression (G.P).


𝑎1 = 3𝑘
𝑎2 = 5𝑘 − 6
𝑎3 = 6𝑘 − 4


Expression for Common Ratio (𝑟) in a Geometric Progression (G.P) is;
𝑎𝑛
𝑟=
𝑎𝑛−1
Hence;
𝑎𝑛
𝑟=
𝑎𝑛−1

, CIE/GCE/AS/Math/P1/19/Nov/13/Q#9
𝑎3 𝑎3
𝑟= =
𝑎3−1 𝑎2


𝑎2 𝑎2
𝑟= =
𝑎2−1 𝑎1


𝑎3 𝑎2
=
𝑎2 𝑎1


6𝑘 − 4 5𝑘 − 6
=
5𝑘 − 6 3𝑘


(6𝑘 − 4)3𝑘 = (5𝑘 − 6)2


(5𝑘)2 − 2(5𝑘)(6) + (6)2 = 18𝑘2 − 12𝑘



25𝑘2 − 60𝑘 + 36 = 18𝑘2 − 12𝑘



7𝑘2 − 48𝑘 + 36 = 0




ii.


We are required to solve the following equation obtained in (i);


7𝑘2 − 48𝑘 + 36 = 0

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