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    Question

    If, a 3  + b 3  = 485 and a + b =

    5, then find the value of (1/a) + (1/b) .
    A -5/24 Correct Answer Incorrect Answer
    B -9/34 Correct Answer Incorrect Answer
    C -7/25 Correct Answer Incorrect Answer
    D -8/25 Correct Answer Incorrect Answer

    Solution

    We know that, (x + y)  3  = x 3  + y 3  + 3xy X (x + y)

    So, (a + b)  3  = a 3  + b 3  + 3ab X (a + b)

    Or, 5 3  = 485 + 3 X ab X 5

    Or, 15ab = 125 - 485

    Or, ab = - (360/15)

    So, ab = - 24

    (1/a) + (1/b) = (a + b) ÷ ab

    = 5 ÷ ( - 24) = - (5/24)

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