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NatalieLove

  • 4 years ago

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

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  1. myininaya
    • 4 years ago
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    Do you see that the numbers are increasing by 8?

  2. NatalieLove
    • 4 years ago
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    yes

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

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

  5. NatalieLove
    • 4 years ago
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    Im still not getting the correct answer

  6. Zarkon
    • 4 years ago
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    whay are people using d=8

  7. Zarkon
    • 4 years ago
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    *why

  8. NatalieLove
    • 4 years ago
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    i dont know im gonna guess

  9. FoolForMath
    • 4 years ago
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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 \(-3230 \)

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

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

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

  13. FoolForMath
    • 4 years ago
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    Glad to help :)

  14. myininaya
    • 4 years ago
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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

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