5
$\begingroup$

PROBLEM
Let $\Gamma$ be the circumcircle of $∆ABC$. Let $D$ be a point on the side $BC$. The tangent to $\Gamma$ at $A$ intersects the parallel line to $BA$ through $D$ at point $E$. The segment $CE$ intersects $\Gamma$ again at $F$. Suppose $B$, $D$, $F$, $E$ are concyclic. Prove that $AC, BF, DE$ are concurrent.


MY APPROACH
Let $X$ be some point on tangent line then, $$\angle XAB=\angle ACB=\angle ACD \quad(1)$$ Since $AB ||DE$, we cany say that $$\angle AED=\angle XAB \quad(2)$$ From $(1)$ and $(2)$, we can say that $$\angle AED=\angle ACD$$ Therefore $ADCE$ is cyclic.

Consider the Circles of $BDEF$, Circles of $ADCE$ and Circumcircle of $∆ABC$. We see that $AC,BF$ and $DE$ are the radical axes of these circles which means they are concurrent at the radical Centre.

Hence, Proved!


But APMO-$2020$ haven't included my solution in their Official Answers. Is something wrong with my Proof?


DIAGRAM Diagram

$\endgroup$
4
  • $\begingroup$ @Rohit Singh thanks for editing $\endgroup$ Commented May 12, 2022 at 15:53
  • $\begingroup$ Nice proof! I'll note that you can make it slightly simpler by showing $A$, $D$, $C$, $E$ are concyclic via $\angle CDE\cong\angle CAE$, using very similar arguments. This eliminates the need to introduce the point $X$. (BTW: Introducing $X$ requires a little more specificity than "some point on [the] tangent line". If $X$ were on the other side of $A$ than you have drawn, then your argument would need a slight adjustment.) $\endgroup$ Commented May 12, 2022 at 16:25
  • $\begingroup$ @Blue yeah I think it's more simple,Thanks $\endgroup$ Commented May 12, 2022 at 16:31
  • $\begingroup$ Your figure is correct, but $D$ looks to be the foot of the altitude issued from $A$... $\endgroup$ Commented May 12, 2022 at 16:44

2 Answers 2

3
$\begingroup$

You have a beautiful, neat proof for this problem, and it is absolutely correct, in fact it looks much more neat than the official solutions. There is nothing wrong with your solution.

But when exams are created, the creators do not know all the possible solutions to a problem, and they don't list all of them. However, as we try to solve them we can discover many new proofs which were not originally given as official solutions.

This is your own solution to the problem, and it doesn't have to be among the official solutions. Hope this helps :)

$\endgroup$
0
2
$\begingroup$

Your proof is fine. By the way, it use with great ingenuity the hypothesis that e 𝐵, 𝐷, 𝐹, 𝐸 are concyclic via the property of radical center.

$\endgroup$
0

You must log in to answer this question.

Start asking to get answers

Find the answer to your question by asking.

Ask question

Explore related questions

See similar questions with these tags.