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WAEC Further Mathematics 2008 Question 19

Question 19

Binomial Theorem

The coefficient of the 5th term in the binomial expansion of (1 + kx)8, in ascending powers of x is 35/8. Find the value of constant k.

  1. A.2
  2. B.1/2
  3. C.−1/2
  4. D.−2
Show the answer and explanation

Answer: B (1/2)

Explanation

The 5th term is 8C4(kx)4 = 70k4x4. So 70k4 = 35/8, k4 = 1/16 and k = 1/2 (k = −1/2 also fits).