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JAMB Mathematics 1983 Question 18

Question 18

Graphs of Functions

Which of the following equations represents the above graph?

Diagram for this question
  1. A.y = 1 + 2x + 3x2
  2. B.y = 1 + 2x + 3x2
  3. C.y = 1 + 2x − 3x2
  4. D.y = 1 − 2x − 3x2
  5. E.y = 3x2 + 2x − 1.
Show the answer and explanation

Answer: D (y = 1 − 2x − 3x2)

Explanation

Roots −1 and ⅓ give the factors (x + 1)(3x − 1) = 3x² + 2x − 1. The curve is an upside-down parabola through (0, 1), so y = −(3x² + 2x − 1) = 1 − 2x − 3x².