arithmetic progression

posted by bello

An arithmetic progression has 3 as its first term. Also, the sum of the first 8 terms is twice the sum of the first 5 terms. Find the common difference.

  1. Scott

    2[(5 * 3) + 10d] = (8 * 3) + 28d

  2. Reiny

    sum(8) = 4(2a + 7d)
    = 8a + 28d = 24 + 28d
    sum(5) = (5/2)(2a + 4d)
    = 5a + 10d
    = 15 + 10d

    24+28d = 2(15+10d)
    solve for d, let me know what you get

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