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JAMB Mathematics 2014 Question 20

Question 20

Progressions

What is the common ratio of the G.P. (√10 + √5) + (√10 + 2√5) + …?

  1. A.5
  2. B.√2
  3. C.√5
  4. D.3
Show the answer and explanation

Answer: B (√2)

Explanation

r = (√10 + 2√5)/(√10 + √5). Multiply top and bottom by (√10 − √5): the top is 10 − √50 + 2√50 − 10 = √50 = 5√2 and the bottom is 5, so r = √2.