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Last updated at May 29, 2018 by Teachoo

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Misc 10 The sum of three numbers in G.P. is 56. If we subtract 1, 7, 21 from these numbers in that order, we obtain an arithmetic progression. Find the numbers. Let the numbers in G.P. be a, ar, and ar2. It is given that Sum of these number is 56 a + ar + ar2 = 56 ar2 = 56 a ar Also, When 1, 7, 21 subtracting from these number respectively, the new numbers are in AP So, (a 1) , (ar 7) , (ar2 21) are in AP Common difference is same (ar 7) (a 1) = (ar2 21) (ar 7) ar a 6 = ar2 ar 14 ar2 ar ar + a 14 + 6 = 0 ar2 2ar + a 8 = 0 From (1) putting ar2 = 56 a ar (56 a ar) 2ar + a 8 = 0 56 a ar 2ar + a 8 = 0 -a + a ar 2ar + 56 8 = 0 0 3ar + 48 = 0 -3ar = -48 ar = ( 48)/( 3) ar = 16 a = 16/ Putting a = (16 )/ in (1) a + ar + ar2 = 56 a(1 + r + r2) = 56 16/ (1 + r + r2) = 56 (16 (1+ + 2))/ = 56 (16 (1+ + 2))/ = 56 16(1 + r + r2) = 56r 16 + 16r + 16r2 56r = 0 16r2 + 16r 56r + 16 = 0 8(2r2 5r + 2) = 0 (2r2 5r + 2) = 0/8 2r2 5r + 2 = 0 2r2 4r r + 2 = 0 2r(r 2) 1(r 2) = 0 (2r 1)(r 2) = 0 2r 1 = 0 or r 2 = 0 2r = 1 or r = 2 r = 1/2 or r = 2 Now, finding numbers Thus the numbers are 8, 16, 32 for r = 1/2 & a =32 & 32, 16, 8 for r = 2 & a = 8

Miscellaneous

Misc 1

Misc 2

Misc 3 Important

Misc 4

Misc 5

Misc 6

Misc 7 Important

Misc 8

Misc 9

Misc 10 You are here

Misc 11

Misc 12

Misc 13

Misc 14

Misc 15

Misc 16 Important

Misc 17

Misc 18

Misc 19 Important

Misc 20

Misc 21

Misc 22

Misc 23

Misc 24

Misc 25 Important

Misc 26

Misc 27

Misc 28 Important

Misc 29

Misc 30

Misc 31

Misc 32 Important

Chapter 9 Class 11 Sequences and Series

Serial order wise

About the Author

Davneet Singh

Davneet Singh is a graduate from Indian Institute of Technology, Kanpur. He has been teaching from the past 9 years. He provides courses for Maths and Science at Teachoo.