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    Question

    If x 4  – 83x 2  + 1 = 0, then

    the value of x 3  - x -3  can be:
    A 756 Correct Answer Incorrect Answer
    B 748 Correct Answer Incorrect Answer
    C 742 Correct Answer Incorrect Answer
    D 744 Correct Answer Incorrect Answer

    Solution

    x 4  – 83x 2  + 1 = 0

    So, (x 2  + 1/x 2 ) = 83

    So, (x – 1/x) = √(83 – 2) = 9

    So, x 3  - x -3  = 729 + 3 × 9 = 756

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