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Find the arithmetic sequence whose fifth term = 21, and the tenth term = 3 times the second term

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  1. by our definitions:
    a + 4d = 21 ---> a = 21-4d **

    a+ 9d = 3(a+d)
    a + 9d = 3a + 3d
    2a = 6d
    a = 3d ***

    equate ** and ***
    3d = 21-4d
    7d = 21

    carry on

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