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NatalieLove

What is the sum of the arithmetic sequence 137, 125, 113 …, if there are 38 terms?

  • one year ago
  • one year ago

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  1. myininaya
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    Do you see that the numbers are increasing by 8?

    • one year ago
  2. NatalieLove
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    yes

    • one year ago
  3. mariomintchev
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    i would do it the old school way and keep subtracting 12 until you get 38 terms, then add lol

    • one year ago
  4. myininaya
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    \[\sum_{i=0}^{37}(137+8i)=137+8(0)+\sum_{i=1}^{37}(137+8i)\] Now use \[\sum_{i=1}^{n}i=\frac{n(n+1)}{2} \text{ and } \sum_{i=1}^{n}c=cn\]

    • one year ago
  5. NatalieLove
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    Im still not getting the correct answer

    • one year ago
  6. Zarkon
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    whay are people using d=8

    • one year ago
  7. Zarkon
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    *why

    • one year ago
  8. NatalieLove
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    i dont know im gonna guess

    • one year ago
  9. FoolForMath
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    Whenever you need to find the sum of an arithmetic series use this formula: \[ \frac n 2 \left( 2a+ (n-1) \right)\times d) \tag{1}\] a= first term n= number of terms d= common difference Here, \(n= 38, a = 137 \) and \( d = -12\). Substituting these in \((1)\), and assuming my algebra is right you answer should be \(-3230 \)

    • one year ago
  10. NatalieLove
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    I know but the answer does not show up in the multiple choose

    • one year ago
  11. FoolForMath
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    @NatalieLove: Revert if you need help in proving that formula.

    • one year ago
  12. NatalieLove
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    We are doing the same steps the test its incorrect. It does not show up as a choose Thank You

    • one year ago
  13. FoolForMath
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    Glad to help :)

    • one year ago
  14. myininaya
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    omg lol I said increasing by 8 and then continue to think the thingy was increasing by 8 sometimes i wonder about myself lol

    • one year ago
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