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    Question

    The sum of the total surface areas of two cubes is 1392

    cm2. If the product of the length of their edges is 98 cm2, then find the difference (in cm) between the lengths of their edges.
    A 4 cm Correct Answer Incorrect Answer
    B 6 cm Correct Answer Incorrect Answer
    C 5 cm Correct Answer Incorrect Answer
    D 7 cm Correct Answer Incorrect Answer

    Solution

    Let the length of the edges of the two cubes be ‘a’ cm and ‘b’ cm, respectively According to the question, 6a2 + 6b2 = 1392 Or, a2 + b2 = 1392/6 = 232 And, ab = 98 Using the formula,   x2 + y2 = (x – y)2 + 2xy 232 = (a – b)2 + 2 × 98 Or, (a – b)2 = 232 – 196 Or, (a – b)2 = 36 Or, a – b = ±6 So desired difference = 6 cm 

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