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Three circles intersecting at one point

It is straightforward to see that $A',B',C'$ are reflections of the circumcenter $O$ with respect to $BC, CA,AB$. Therefore, the center of $(AOA')$ is just the intersection of the mediatrix of $OA$ wi …
Beni Bogosel's user avatar
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1 vote

Intersection point of three circles

Switching the roles of $ABC$ and $A'B'C'$, consider the circles passing through the vertices of a triangle $A,B,C$, midpoints of the opposite sides $A',B',C'$ and the circumcenter $O$. It is straightf …
Beni Bogosel's user avatar
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A generalization of Napoleon's theorem

Notice that triangles $ACD, AEB, FCB$ are similar. Working out the ratios of the sides and the angles one can see that: Triangles $AI_1I_3$ and $ACE$ are similar. … Triangles $BI_1I_2$ and $BEC$ are similar. Rotating $I_1I_2$ around $B$ with angle $\frac{1}{2}\angle FBC$ makes $I_1 I_2$ parallel to $CE$. …
Beni Bogosel's user avatar
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2 votes
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Important lines in triangle - reverse problem

It is known that if three numbers $x,y,z$ are the lengths of the edges of some triangle, then there exists a triangle with medians of length $x,y,z$. Also, if $x,y,z>0$ (no condition imposed) there ex …