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    Question

    If 4x + 1/5x = 4, then find value of 125x³ + 1/64x³

    A 225/4 Correct Answer Incorrect Answer
    B 425/4 Correct Answer Incorrect Answer
    C 725/4 Correct Answer Incorrect Answer
    D 325/4 Correct Answer Incorrect Answer

    Solution

    4x + 1/5x = 4 By multiplying 5/4 , 4x × 5/4 + 1/5x × 5/4 = 4 × 5/4 5x + 1/4x = 5 on cubing both the sides (5x+1/4x)3= (5)³ 125x³ + 1/64x³ + 3 × 5x × 1/4x (5x+1/4x) = 125 125x³ + 1/64x³ + (3×5)/4 × 5 = 125 125x³ + 1/64x³ + 75/4 = 125 125x³ + 1/64x³ = 125 - 75/4 125x³ + 1/64x³ = (500-75)/4 125x³ + 1/64x³ = 425/4

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