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JAMB Mathematics 1983 Question 14

Question 14

Variation

y varies partly as the square of x and partly as the inverse of the square root of x. Write down the expression for y if y = 2 when x = 1 and y = 6 when x = 4.

  1. A.y = 10x2/31 + 52/(31√x)
  2. B.y = x2 + 1/√x
  3. C.y = x2 + 1/x
  4. D.y = x2/31 + 1/(31√x)
  5. E.y = 10/31 (x2 + 1/√x)
Show the answer and explanation

Answer: A (y = 10x2/31 + 52/(31√x))

Explanation

y = ax² + b/√x. x = 1 gives a + b = 2; x = 4 gives 16a + b/2 = 6, i.e. 32a + b = 12. Subtracting: 31a = 10, so a = 10/31 and b = 52/31.