Point point

Geometry Level 1

Given two distinct points A A and B B in the plane, how many distinct points C C are there on the same plane such that A B C \triangle ABC is an equilateral triangle?


The answer is 2.

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8 solutions

If A B C \triangle ABC is an equilateral triangle, it satisfies A B = A C AB = AC and B A = B C BA = BC . The set of points C C that satisfy the first condition is a circumference with center A A and radius A B AB , while the set of points C C that satisfy the second condition is a circumference with center B B and radius B A BA . Both circumferences intersect at two points, which are the only points C C such that A B C \triangle ABC is equilateral.

Moderator note:

Great approach, using the intersection of 2 circles to clearly mark the 2 possible points of C C .

Only two if you can imagine how it's work, just for example if AB is in line y (x=0) so equilateral triangle that can happen is where C is to right line x (+) and to left line x (-).

Ex. Consider two points in the x axis of the plane would be A=2, B=6. In an equilateral triangle all sides should have same angle. according to this, the point C would be in 4 of x axis. the point can be placed above and below the line A and B So the answer is 2

Souperman Arcena
Jul 29, 2013

so an equilateral triangle sides are equal so c=b=a c=2

Nandana Smart
Jul 29, 2013

The point C can be on either side of line joining A to B . So number of points =2. On one side of the line segment there will be only one point since two rays will intersect once only.

Dishant Shah
Jul 29, 2013

The pts. lie on the perpendicular bisector of line AB at a distance = AB from A and B . you can try it manually also

Victor Carnaúba
Jul 28, 2013

That's easy, we have 1 especific C that we can made a equilateral but is one above and one below, so 2

Joshua Crouch
Jul 28, 2013

If there are two points in a plane that are 1 inch apart, then the other point must be 1 inch apart from the two points. It can be to the left and then reflected to the right. This results in two possible points.

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