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Difficulty: 2/102023 NMTC 2023 (QII-24)

Total number of circles that can pass through 2 pair of points in plane are ________.

Guide / Hint

Hint 1: Consider two points and . Where must the center of a circle passing through and lie?

Hint 2: The center must lie on the perpendicular bisector of the segment . How many points are on a straight line?

Hint 3: Conclude that there are infinitely many possible centers, hence infinitely many circles.

Solution

Step 1 (Geometric Condition): Let the two points in the plane be and . A circle passing through both and must have its center equidistant from and .

Step 2 (Locus of Centers): The set of all points equidistant from and is the perpendicular bisector of the line segment . This bisector is a straight line containing infinitely many points.

Step 3 (Circles Construction): Any point on this perpendicular bisector can be chosen as the center of a circle with radius (which equals ). Since there are infinitely many points on the perpendicular bisector, there are infinitely many distinct circles that can pass through the two points.

Step 4 (Conclusion): The number of circles is infinity.

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    2023 NMTC 2023 QII-24 - Olympiad Math Olympiad Question | Leminno