1955 AMC 12 Problem 28

Attempt Problem 28 of the 1955 AMC 12 below, then check your answer against the professionally curated solution from LIVE by Po-Shen Loh. You can also try the full timed exam, view all 1955 AMC 12 solutions, or check the answer key.

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28.

On the same set of axes are drawn the graph of y=ax2+bx+cy=ax^2+bx+c and the graph of the equation obtained by replacing xx by −x-x in the given equation. If b≠0b\ne0 and c≠0c\ne0 these two graphs intersect:
in two points, one on the xx-axis and one on the yy-axis
in one point located on neither axis
only at the origin
in one point on the xx-axis
in one point on the yy-axis
Answer: E
Concepts:parabola reflectionsimultaneous equationssymmetry
Small hint:
The reflected graph is y=ax2−bx+cy=ax^2-bx+c
Big hint:
Set the two expressions for yy equal and use b≠0b\ne0
Solution:
Replacing xx by −x-x gives y=ax2−bx+c.y=ax^2-bx+c. At an intersection, ax2+bx+c=ax2−bx+c, ax^2+bx+c=ax^2-bx+c, so 2bx=0.2bx=0. Since b≠0,b\ne0, we must have x=0,x=0, and then y=c≠0.y=c\ne0. There is exactly one intersection, (0,c),(0,c), on the yy-axis.
Thus, the correct answer is E.
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