Question

    If A = (7)1/3 + (7) -1/3, then the

    value of {3A3 – 9A} will be:
    A 200/7 Correct Answer Incorrect Answer
    B 50/7 Correct Answer Incorrect Answer
    C 100/7 Correct Answer Incorrect Answer
    D 150/7 Correct Answer Incorrect Answer

    Solution

    A = (7)1/3 + (7) -1/3,   Cubing both side A3 = 7 + (1/7) + 3 × (7)1/3 × (7) -1/3 × {(7)1/3 + (7) -1/3}   Or, A3 = 7 + (1/7) + 3A Or, A3 – 3A = 50/7 And, 3A3 – 9A = (50 × 3)/7 = 150/7  

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