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The binary operation □ is defined as a □ b = a + b + ab, where a and b are any two real numbers. The value of the identity element of this operation, defined as the number z such that a □ z = a for any a, is A. 0 B. 1 C. 2 D. 10

GATE 2016 · General Aptitude · Number Theory · easy

Answer: z = 0

  1. Apply the operation definition to a □ z = a: a □ z = a + z + az. Setting this equal to a: a + z + az = a.
  2. Solve for z: z + az = 0 => z(1 + a) = 0. For this to hold for ALL real numbers a (including values where a != -1), we need z = 0.
  3. Verify: a □ 0 = a: a □ 0 = a + 0 + 0 = a. Confirmed, z = 0 is the identity element.