1970 AMC 12 Problem 26

Attempt Problem 26 of the 1970 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 1970 AMC 12 solutions, or check the answer key.

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26.

The number of distinct points in the xyxy-plane common to the graphs of (x+y−5)(2x−3y+5)=0 (x+y-5)(2x-3y+5)=0 and (x−y+1)(3x+2y−12)=0 (x-y+1)(3x+2y-12)=0 is:
00
11
22
33
44
infinite
Answer: B
Concepts:system of equationszero product propertycounting intersections
Small hint:
Each product equation represents a union of two lines
Big hint:
Check the intersection of the two lines in each pair before considering all four pairings
Solution:
The first graph is the union of x+y=5,2x−3y=−5, x+y=5,\qquad 2x-3y=-5, and those two lines meet at (2,3).(2,3). The second graph is the union of x−y=−1,3x+2y=12, x-y=-1,\qquad 3x+2y=12, and these also meet at (2,3).(2,3). In fact, substituting (2,3)(2,3) satisfies all four line equations. Since the four lines have distinct slopes, no other point can belong to a line from each graph. There is exactly one common point.
Therefore, the correct answer is B.
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