Let $\triangle ABC$ be an acute-angled triangle; $L$, $M$, $N$ be the feet of perpendiculars respectively from $A$, $B$, $C$ to the opposite sides; $D$, $E$, $F$ be the midpoints of the sides $BC$, $CA$, $AB$ respectively; and $I_1, I_2, I_3$ be the ex-centers of $\triangle ABC$. Then, which of the following is true ?

(A) the ortho-center of $\triangle ABC$ is the in-center of $\triangle LMN$.

(B) The circum-center of $\triangle ABC$ is the ortho-center of $\triangle DEF$.

(C) The in-center of $\triangle ABC$ is the ortho-center of $\triangle I_1 I_2 I_3$.

(D) The centroid of $\triangle ABC$ is the centroid of $\triangle DEF$

No idea how to proceed.

  • 1
    $\begingroup$ Try drawing an acute scalene triangle and seeing if any of the four are feasible in your example. If one seems to be true, use the picture to try to work out a proof. The picture doesn't amount to a proof, but it can give helpful clues. $\endgroup$
    – rschwieb
    May 14, 2014 at 16:42
  • 1
    $\begingroup$ And have you learned any useful facts about these points of interest in your course? $\endgroup$
    – rschwieb
    May 14, 2014 at 16:47
  • $\begingroup$ You can Google these things, for e.g 'orthic triangle'. $\endgroup$
    – Sawarnik
    May 14, 2014 at 17:14
  • $\begingroup$ Used geogebra. It seems all are true! $\endgroup$
    – Ruddie
    May 16, 2014 at 14:59

1 Answer 1


This image has been obtained using geogebra. It seems all are true!

enter image description here


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