Nine point circle

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Also known as Euler's circle or Feuerbach's circle, as its name introduces itself that the nine point circle passes nine points, which are a given triangle $\triangle ABC$'s feet of altitude dropped from three vertices $A,B$, and $C$, usually denoted as $H_A,H_B,H_C$, and midpoints of three sides, as $M_A,M_B,M_C$, and three Euler points $E_A,E_B,E_C$.

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