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koli123able

  • 3 years ago

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.

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  1. stgreen
    • 3 years ago
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    use an=a1*r^(n-1)

  2. stgreen
    • 3 years ago
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    where an=48 and a1=7

  3. stgreen
    • 3 years ago
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    *448

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

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

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

  7. koli123able
    • 3 years ago
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    i am stuck here

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

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

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

  11. koli123able
    • 3 years ago
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    The answer should come r=2 and n=7

  12. stgreen
    • 3 years ago
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    yeah hold on lemme write

  13. koli123able
    • 3 years ago
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    ok

  14. stgreen
    • 3 years ago
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    |dw:1356937412146:dw|

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

  16. stgreen
    • 3 years ago
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    got it??

  17. koli123able
    • 3 years ago
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    yes

  18. stgreen
    • 3 years ago
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    good

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