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Railways PYQCBT 1 2020

RRB NTPC · Question 75

If 2x + 5y = 12, xy = 2, find the value of: 8x^3 + 125y^3

  1. 1008

  2. 1208

  3. 1028

  4. 1018

Show answer & explanation

Answer: (A) 1008

Explanation

(2x + 5y)³ = 8x³ + 125y³ + 3 × 2x × 5y × (2x + 5y). So 1728 = 8x³ + 125y³ + 30 × 2 × 12, giving 8x³ + 125y³ = 1728 − 720 = 1008.