1956 AMC 12 Problem 29

Attempt Problem 29 of the 1956 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 1956 AMC 12 solutions, or check the answer key.

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29.

The points of intersection of xy=12xy=12 and x2+y2=25x^2+y^2=25 are joined in succession. The resulting figure is:
a straight line
an equilateral triangle
a parallelogram
a rectangle
a square
Answer: D
Concepts:systems of equationscoordinate geometryquadrilaterals
Small hint:
Use (x+y)2=x2+y2+2xy(x+y)^2=x^2+y^2+2xy to find the possible sums x+yx+y
Big hint:
The four intersection points are permutations and negatives of (3,4)(3,4)
Solution:
At an intersection, (x+y)2=x2+y2+2xy=25+24=49, \begin{aligned} (x+y)^2&=x^2+y^2+2xy\\ &=25+24=49, \end{aligned} so x+y=±7.x+y=\pm7. Together with xy=12,xy=12, this gives the four points (3,4), (4,3),(−3,−4), (−4,−3). \begin{aligned} &(3,4),\ (4,3),\\ &(-3,-4),\ (-4,-3). \end{aligned} In their cyclic order around the circle, adjacent side vectors are perpendicular, so the quadrilateral is a rectangle. Its adjacent side lengths are unequal, so it is not a square.
Thus, the correct answer is D.
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