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WAEC Further Mathematics 2019 Question 6

Question 6

Binary Operations

An operation (*) is defined on the set T = {−1, 0, …, 5} by x * y = x + y − xy. Which of the following operation(s) will give an image which is an element of T? I. 2(*)5 II. 3(*)2 III. 3(*)4

  1. A.I only
  2. B.II only
  3. C.I and III only
  4. D.II and III only
Show the answer and explanation

Answer: B (II only)

Explanation

2 * 5 = 7 − 10 = −3 (not in T); 3 * 2 = 5 − 6 = −1 (in T); 3 * 4 = 7 − 12 = −5 (not in T). Only II.