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Sequence math problem

  1. Sep 18, 2005 #1

    IB

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    Please help...

    Given U3 = 0
    S15 = -300
    How can you find d? And U1?

    (Instead of "T", we use "U" for "term", and d is the common difference)

    Thanks!
     
    Last edited: Sep 18, 2005
  2. jcsd
  3. Sep 18, 2005 #2

    IB

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    No one can solve this problem? :(
     
  4. Sep 18, 2005 #3
    two simultaneous equations.
    one equation for each information given.
    solve the equations for a and d.
     
  5. Sep 18, 2005 #4

    IB

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    Thanks a lot!
     
  6. Sep 18, 2005 #5
    Set up the 2 equations:

    Sn = n/2[2a + (n-1)d]
    Un = a + (n-1)d

    Substitute known values:

    -300 = 15/2 [2a + (15-1)d]
    -40 = 2a + 14d
    -40 - 14d = 2a
    a = -20 - 7d

    0 = a + (3-1)d
    0 = a + 2d
    a = -2d

    Set them equal to each other and solve:

    -20 - 7d = -2d
    -20 = 5d
    d = -4

    there's your d value bro....i'm sure you can do the rest! :approve:
     
  7. Sep 18, 2005 #6

    IB

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    Yup, my bro's so smart! :smile:
     
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