Non-Obvious Quadratic

If the points (1, y1) and (-1, y2) lie on the graph of

y = ax2 + bx + c, and the two points are such that y1y2 = -6, then b equals:

  1. -3
  2. 0
  3. 3
  4. ac
  5. a + c/2


Show/Hide Solution

y = ax2 + bx + c

(1, y1):  a + b + c  =  y1
(-1, y2):  ab + c  =  y2
  0 + 2b + 0  =  y1y2 = -6
  2b  =  -6
  b  =  -3.

The answer is (a).


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