Triangle $ABC$ has perimeter $4$. Points $X$ and $Y$ lies on rays $AB$ and $AC$, respectively, such that $AX = AY = 1$. Segments $BC$ and $XY$ intersect at point $M$. Prove that the perimeter of either $\bigtriangleup ABM$ or $\bigtriangleup ACM$ is $2$.

[I like this problem ]

## All Russian Math Olympiad 2010

### All Russian Math Olympiad 2010

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- ahmedittihad
**Posts:**147**Joined:**Mon Mar 28, 2016 6:21 pm

### Re: All Russian Math Olympiad 2010

Assume, $AB<AC$.

Reflect $A$ over $X$ and $Y$ to get $K$ and $L$ respectively. We know that the length of the tangent from $A$ to the $A$-excircle is half the perimeter. So, we get, $K$ and $L$ are the touchpoints of the $A$-excircle with $AB$ and $AC$.

Let the $A$-excircle meet $BC$ at $Z$.

Now, the wow factor.

We show that $MZ=AM$.

As $X$ and $Y$ are the midpoints of $AK$ and $AL$, $XY$ is the radical axis of the $A$-excircle and the zero radius circle at $A$. And $M$ lies on the radical axis. Yielding $MZ=AM$.

Now, $AK=2=AB+BK=AB+BZ=AB+BM+MZ=AB+BM+AM$.

The other cases are similar too.

Reflect $A$ over $X$ and $Y$ to get $K$ and $L$ respectively. We know that the length of the tangent from $A$ to the $A$-excircle is half the perimeter. So, we get, $K$ and $L$ are the touchpoints of the $A$-excircle with $AB$ and $AC$.

Let the $A$-excircle meet $BC$ at $Z$.

Now, the wow factor.

We show that $MZ=AM$.

As $X$ and $Y$ are the midpoints of $AK$ and $AL$, $XY$ is the radical axis of the $A$-excircle and the zero radius circle at $A$. And $M$ lies on the radical axis. Yielding $MZ=AM$.

Now, $AK=2=AB+BK=AB+BZ=AB+BM+MZ=AB+BM+AM$.

The other cases are similar too.

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- Atonu Roy Chowdhury
**Posts:**40**Joined:**Fri Aug 05, 2016 7:57 pm**Location:**Chittagong, Bangladesh

### Re: All Russian Math Olympiad 2010

Well, I first missed the point that segments $BC$ and $XY$ intersects at $M$. After noticing this the problem is easy. My solution is similar to Ittihad's.

WLOG $AB>AC$. Take the $A$-excircle of $\triangle ABC$. it touches $BC$ at $D$ and the extensions of $AB$ and $AC$ at $E$ and $F$ respectively. $AF = 2$. $XY$ is the rad axis of the excircle and point circle $A$. So, $AM=MD$. The rest is trivial.

WLOG $AB>AC$. Take the $A$-excircle of $\triangle ABC$. it touches $BC$ at $D$ and the extensions of $AB$ and $AC$ at $E$ and $F$ respectively. $AF = 2$. $XY$ is the rad axis of the excircle and point circle $A$. So, $AM=MD$. The rest is trivial.