16Hit1600
Advanced MathHardCounting intersections of a line and a parabola

Advanced Math practice question

In the xy-plane, how many points do the graphs of y = x² + 1 and y = x have in common?

  1. A. ZeroCorrect
  2. B. Exactly one
  3. C. Exactly two
  4. D. Infinitely many

Answer: A. Zero

Set them equal: x² + 1 = x, so x² − x + 1 = 0. The discriminant is (−1)² − 4(1)(1) = −3, which is negative, so there are no real solutions — the graphs never meet. (The parabola's lowest point is (0, 1), which already sits above the line there.)

Why the other answers are wrong

B. Exactly one
Exactly one point would need a discriminant of zero; here it's −3.
C. Exactly two
Two points would need a positive discriminant. −3 is negative.
D. Infinitely many
A line and a parabola can share at most two points; infinitely many is impossible.

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