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JAMB Mathematics 1978 Question 9

Question 9

Surds

Find a square root of 170 − 20√30

  1. A.2√10 − 5
  2. B.3√5 − 8√6
  3. C.2√5 − 5√6
  4. D.5√5 − 2√6
  5. E.5√10 − 2√3
Show the answer and explanation

Answer: C (2√5 − 5√6)

Explanation

(2√5 − 5√6)² = 20 + 150 − 2(2√5)(5√6) = 170 − 20√30, so 2√5 − 5√6 is a square root (the other one is 5√6 − 2√5).