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arithmetic

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if 6 times the sixth term is equal to 15 times the fifteenth term ,find its 21st term

  • arithmetic sequence -

    6(a+5d) = 15(a+14d)
    6a + 30d = 15a + 210d
    9a = -180d
    a = -20d

    a + 20d = -20d + 20d = 0

  • arithmetic -

    24

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