AIME 2003 I · 第 15 题

Solution 1

In the following, let the name of a point represent the mass located there. Since we are looking for a ratio, we assume that $AB=120$, $BC=169$, and $CA=260$ in order to simplify our computations.

First, reflect point $F$ over angle bisector $BD$ to a point $F'$.

As $BD$ is an angle bisector of both triangles $BAC$ and $BF'F$, we know that $F'$ lies on $AB$. We can now balance triangle $BF'C$ at point $D$ using mass points.

By the Angle Bisector Theorem, we can place mass points on $C,D,A$ of $120,\,289,\,169$ respectively. Thus, a mass of $\frac {289}{2}$ belongs at both $F$ and $F'$ because BD is a median of triangle $BF'F$ . Therefore, $CB/FB=\frac{289}{240}$.

Now, we reassign mass points to determine $FE/FD$. This setup involves $\triangle CFD$ and transversal $MEB$. For simplicity, put masses of $240$ and $289$ at $C$ and $F$ respectively. To find the mass we should put at $D$, we compute $CM/MD$. Applying the Angle Bisector Theorem again and using the fact $M$ is a midpoint of $AC$, we find

$$\frac {MD}{CM} = \frac {\frac{169}{289}\cdot 260 - 130}{130} = \frac {49}{289}$$ At this point we could find the mass at $D$ but it's unnecessary.

$$\frac {DE}{EF} = \frac {F}{D} = \frac {F}{C}\cdot\frac {C}{D} = \frac {289}{240}\cdot\frac {49}{289} = \boxed{\frac {49}{240}}$$ and the answer is $49 + 240 = \boxed{289}$.

Solution 2

By the Angle Bisector Theorem, we know that $[CBD]=\frac{169}{289}[ABC]$. Therefore, by finding the area of triangle $CBD$, we see that

$$\frac{507\cdot BD}{2}\sin\frac{B}{2}=\frac{169}{289}[ABC].$$ Solving for $BD$ yields

$$BD=\frac{2[ABC]}{3\cdot289\sin\frac{B}{2}}.$$ Furthermore, $\cos\frac{B}{2}=\frac{BD}{BF}$, so

$$BF=\frac{BD}{\cos\frac{B}{2}}=\frac{2[ABC]}{3\cdot289\sin\frac{B}{2}\cos\frac{B}{2}}.$$ Now by the identity $2\sin\frac{B}{2}\cos\frac{B}{2}=\sin B$, we get

$$BF=\frac{4[ABC]}{3\cdot289\sin B}.$$ But then $[ABC]=\frac{360\cdot 507}{2}\sin B$, so $BF=\frac{240}{289}\cdot 507$. Thus $BF:FC=240:49$.

Now by the Angle Bisector Theorem, $CD=\frac{169}{289}\cdot 780$, and we know that $MC=\frac{1}{2}\cdot 780$ so $DM:MC=\frac{169}{289}-\frac{1}{2}:\frac{1}{2}=49:289$.

We can now use mass points on triangle CBD. Assign a mass of $240\cdot 49$ to point $C$. Then $D$ must have mass $240\cdot 289$ and $B$ must have mass $49\cdot 49$. This gives $F$ a mass of $240\cdot 49+49\cdot 49=289\cdot 49$. Therefore, $DE:EF=\frac{289\cdot 49}{240\cdot 289}=\frac{49}{240}$, giving us an answer of $\boxed{289}.$

Solution 3

Let $\angle{DBM}=\theta$ and $\angle{DBC}=\alpha$. Then because $BM$ is a median we have $360\sin{(\alpha+\theta)}=507\sin{(\alpha-\theta)}$. Now we know

$$\sin{(\alpha+\theta)}=\sin{\alpha}\cos{\theta}+\sin{\theta}\cos{\alpha}=\dfrac{DF\cdot BD}{BF\cdot BE}+\dfrac{DE\cdot BD}{BE\cdot BF}=\dfrac{BD(DF+DE)}{BF\cdot BE}$$ Expressing the area of $\triangle{BEF}$ in two ways we have

$$\dfrac{1}{2}BE\cdot BF\sin{(\alpha-\theta)}=\dfrac{1}{2}EF\cdot BD$$ so

$$\sin{(\alpha-\theta)}=\dfrac{EF\cdot BD}{BF\cdot BE}$$ Plugging this in we have

$$\dfrac{360\cdot BD(DF+DE)}{BF\cdot BE}=\dfrac{507\cdot BD\cdot EF}{BF\cdot BE}$$ so $\dfrac{DF+DE}{EF}=\dfrac{507}{360}$. But $DF=DE+EF$, so this simplifies to $1+\dfrac{2DE}{EF}=\dfrac{507}{360}=\dfrac{169}{120}$, and thus $\dfrac{DE}{EF}=\dfrac{49}{240}$, and $m+n=\boxed{289}$.

Solution 4 (Overpowered Projective Geometry!!)

Firstly, angle bisector theorem yields $\frac{CD}{AD} = \frac{507}{360} = \frac{169}{120}$. We're given that $AM=MC$. Therefore, the cross ratio

$$(A,C;M,D) = \frac{AM(CD)}{AD(MC)} = \frac{169}{120}$$ We need a fourth point for this cross ratio to be useful, so reflect point $F$ over angle bisector $BD$ to a point $F'$. Then $\triangle BFF'$ is isosceles and $BD$ is an altitude so $DF = DF'$. Therefore,

$$(A,C;M,D) = (F,F';D,E) \implies \frac{FD(EF')}{EF(DF')} = \frac{EF'}{EF} = \frac{169}{120}$$ All that's left is to fiddle around with the ratios:

$$\frac{EF'}{EF} = \frac{ED+DF'}{EF} = \frac{EF+2DE}{EF} = 1\ +\ 2\left(\frac{DE}{EF}\right) \implies \frac{DE}{EF} = \frac{49}{240} \implies \boxed{289}$$

Solution 5 (Menelaus + Mass Points)

Extend $DF$ to intersect with the extension of $AB$ at $G$. Notice that $\triangle{BDF} \cong \triangle{BDG}$, so $GD=DF$. We now use Menelaus on $\triangle{GBF}$, as $A$, $D$, and $C$ are collinear; this gives us $\frac{GA}{BA} \cdot \frac{BC}{FC} \cdot \frac{DF}{GD}=1$. As $GD=DF$, we have $\frac{GA}{AB}=\frac{FC}{BC}$, hence $\frac{GA}{120}=\frac{FC}{169}$. Reflect $G$ over $A$ to $G'$. Note that $\frac{G'A}{BA}=\frac{FC}{BC}$, and reflexivity, hence $\triangle{ABC} \sim \triangle{BG'F}$. It's easily concluded from this that $G'F \parallel AC$, hence $G'F \parallel AD$. As $GD=DF$, we have $AD$ is a midsegment of $\triangle{GG'F}$, thus $G'F = 2AD$. We now focus on the ratio $\frac{BF}{BC}$. From similarity, we have $\frac{BF}{BC}=\frac{G'F}{AC}=\frac{2AD}{AC}$. By the angle bisector theorem, we have $AD:DC=120:169$, hence $AD:AC=120:289$, so $\frac{BF}{BC}=\frac{240}{289}$. We now work out the ratio $\frac{DM}{MC}$. $\frac{DM}{MC}=\frac{CD-MC}{MC}=\frac{CD}{MC}-1=\frac{2CD}{AC}-1=\frac{338}{289}-1=\frac{49}{289}$. We now use mass points on $\triangle{BDC}$. We let the mass of $C$ be $240\cdot 49$, so the mass of $B$ is $49 \cdot 49$ and the mass of $D$ is $289\cdot 240$. Hence, the mass of $F$ is $289\cdot 49$, so the ratio $\frac{DE}{EF}=\frac{49}{240}$. Extracting gives $49+240=\boxed{289}.$