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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

AP and GP mix questions

Chapter 9 Class 11 Sequences and Series

Concept wise

- Finding Sequences
- Arithmetic Progression (AP): Formulae based
- Arithmetic Progression (AP): Statement
- Arithmetic Progression (AP): Calculation based/Proofs
- Inserting AP between two numbers
- Arithmetic Mean (AM)
- Geometric Progression(GP): Formulae based
- Geometric Progression(GP): Statement
- Geometric Progression(GP): Calculation based/Proofs
- Inserting GP between two numbers
- Geometric Mean (GM)
- AM and GM (Arithmetic Mean And Geometric mean)
- AP and GP mix questions
- Finding sum from series
- Finding sum from nth number

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.