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    Question

    .If a = `sqrt(11+2sqrt18`    and b

    = `sqrt(11-2sqrt18)` , what is the value of a³ + b³?
    A 86 Correct Answer Incorrect Answer
    B 90 Correct Answer Incorrect Answer
    C 110 Correct Answer Incorrect Answer
    D 135 Correct Answer Incorrect Answer

    Solution

    Given a =`sqrt(11+2sqrt18)`  = `sqrt(9+2+2*3sqrt2)`  =`sqrt((3+sqrt2)^2)`  =`3+sqrt2` Similarly b = `3-sqrt2` ∴ a + b = (3 + √2) + (3 - √2) = 6 a * b = (3 + √2) * (3 - √2) = 9 - 2 = 7 a³ + b³ = (a + b)³ - 3ab(a + b) ⇒ a³ + b³ = 6³ - 3 * 7 * 6 = 216 - 126 ⇒ a³ + b³ = 90

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