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    Question

    If 6x - 3y = 9, and 9xy = 18, then what is the value of 6x

    + 3y?
    A 22 Correct Answer Incorrect Answer
    B 15 Correct Answer Incorrect Answer
    C 19 Correct Answer Incorrect Answer
    D 31 Correct Answer Incorrect Answer

    Solution

    ATQ,

    Given: 6x - 3y = 9

    And 9xy = 18

    Or, xy = 2

    We know that, (a + b)² = a² + b² + 2ab

    And (a - b)² = a² + b² - 2ab

    Or, (a + b)² - (a - b)² = 4ab

    So, (6x + 3y)² - (6x - 3y)² = 4 × 6x × 3y

    Or, (6x + 3y)² - 9² = 144

    Or, (6x + 3y)² = 225

    So, 6x + 3y = ±15

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