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koli123able

In a G.P. , the first term is 7, the last term is 448, and the sum is 889. Find the common ratio and the number of terms.

  • one year ago
  • one year ago

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  1. stgreen
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    use an=a1*r^(n-1)

    • one year ago
  2. stgreen
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    where an=48 and a1=7

    • one year ago
  3. stgreen
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    *448

    • one year ago
  4. koli123able
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    it is coming r^(n-1)=64

    • one year ago
  5. koli123able
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    then i used the sum formula Sn=\[Sn =\frac{ a[r^n-1) }{ r-1 }\]

    • one year ago
  6. stgreen
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    exactly...now you got two equations in two unknowns...so solve for r and n

    • one year ago
  7. koli123able
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    i am stuck here

    • one year ago
  8. stgreen
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    where??tell me the second one

    • one year ago
  9. koli123able
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    \[127=\frac{ r^n-1 }{ r-1 }\]

    • one year ago
  10. stgreen
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    r comes out to be 2.isn't it??

    • one year ago
  11. koli123able
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    The answer should come r=2 and n=7

    • one year ago
  12. stgreen
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    yeah hold on lemme write

    • one year ago
  13. koli123able
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    ok

    • one year ago
  14. stgreen
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    |dw:1356937412146:dw|

    • one year ago
  15. stgreen
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    ^from here you can get r=2....then put r=2 in first equation to get n

    • one year ago
  16. stgreen
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    got it??

    • one year ago
  17. koli123able
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    yes

    • one year ago
  18. stgreen
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    good

    • one year ago
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