Find four numbers in G. P. such that sum of the middle two numbers is 103 and their product is 1. - Mathematics and Statistics

Sum

Find four numbers in G. P. such that sum of the middle two numbers is 10/3 and their product is 1.

Solution

Let the four numbers in G.P. be "a"/"r"^3, "a"/"r", ar, ar3.

According to the second condition,

"a"/"r"^3 ("a"/"r") ("ar")("ar"^3) = 1

∴ a4 = 1
∴ a = 1
According to the first condition,

"a"/"r" + "ar"  10/3

∴ 1/"r" + (1)"r" = 10/3

∴ (1 + "r"^2)/"r" = 10/3

∴ 3 + 3r2 = 10r
∴ 3r2 – 10r + 3 = 0
∴ (r – 3)(3r – 1) = 0

∴ r = 3 or r = 1/3

When r = 3, a = 1

"a"/"r"^3 = 1/(3)^3 = 1/27, "a"/"r" = 1/3, "ar" = 1(3) = 3 and ar3 = 1(3)3 = 27

When r = 1/3, a = 1

"a"/"r"^3 = 1/((1/3)^3) = 27, "a"/"r" = 1/((1/3)) = 3,

"ar" = 1(1/3) = 1/3 and "ar"^3 = 1(1/3)^3 = 1/27

∴ the four numbers in G.P.are

1/27, 1/3, 3, 27 or 27, 3, 1/3, 1/27.

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Balbharati Mathematics and Statistics 1 (Commerce) 11th Standard Maharashtra State Board
Chapter 4 Sequences and Series
Miscellaneous Exercise 4 | Q 5 | Page 64
Balbharati Mathematics and Statistics 1 (Commerce) 11th Standard Maharashtra State Board
Chapter 4 Sequences and Series
Exercise 4.1 | Q 7 | Page 51
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