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    Question

    If 8x3 - 125y3 = (2x - Ay) X

    (Bx2 + 25y2 + Cxy), then find the value of 3 X (2A + 6B) - 2C.
    A 96 Correct Answer Incorrect Answer
    B 64 Correct Answer Incorrect Answer
    C 70 Correct Answer Incorrect Answer
    D 82 Correct Answer Incorrect Answer

    Solution

    8x3 - 125y3 = (2x - Ay) X (Bx2 + 25y2 - Cxy) We know that, a3 - b3 = (a - b) X (a2 + b2 + ab) So, 8x3 - 125y3 = (2x) 3 - (5y) 3 = (2x - 5y) X (4x2 + 25y2 + 2x X 5y) = (2x - 5y) X (4x2 + 25y2 + 10xy) So,(2x - Ay) X (Bx2 + 25y2 + Cxy) = (2x - 5y) X (4x2 + 25y2 + 10xy) On comparing coefficients of LHS and RHS, we get, A = 5, B = 4, and C = 10 So, 3 X (2A + 6B) - 2C = 3 X {2 X 5 + 6 X 4} - 2 X 10 = 3 X (10 + 24) - 20 = 3 X 34 - 20 = 102 - 20 = 82

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