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    Question

    If a3 = 117 + b3 and a = 3 + b,

    then the value of a + b is:
    A ±7 Correct Answer Incorrect Answer
    B ±49 Correct Answer Incorrect Answer
    C ±13 Correct Answer Incorrect Answer

    Solution

    a3 – b3 = 117 and a – b = 3 ⇒ a3 – b3 = (a – b) (a2 + b2 +ab) ⇒ 117 = 3 (a2 + b2 +ab) ⇒ 39 = a2 + b2 +ab ⇒ a2 + b2 = 39 – ab we Know that (a-b)² = a2 +b² - 2ab                         = (a-b)² = 39 – ab – 2ab                         = (a-b)² = 39 – 3ab                   9    = 39 – 3ab = ab = 10 Now use (a+b)² = a2 + b2 + 2ab (a+b)² = 39 – ab + 2ab (a+b)² = 39 + ab = 39 + 10 (a+b)² = 49 = a + b = ±7  

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