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Further mathematics

The twelfth term of alinear sequence is 47 and the sum of The first 3 terms is 21 find the sum of the first 15 terms of the sequence

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  1. a + 11 d = 47 ... 3 a + 33 d =141

    3 a + 2 d = 21

    subtracting equations (to eliminate a) ... 31 d = 120

    find a and d

    plug into the sum formula

    f y i ... it seems like there may be an incorrect value in the problem

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