# Difference between revisions of "2018 AMC 10A Problems/Problem 24"

The following problem is from both the 2018 AMC 12A #18 and 2018 AMC 10A #24, so both problems redirect to this page.

## Problem

Triangle $ABC$ with $AB=50$ and $AC=10$ has area $120$. Let $D$ be the midpoint of $\overline{AB}$, and let $E$ be the midpoint of $\overline{AC}$. The angle bisector of $\angle BAC$ intersects $\overline{DE}$ and $\overline{BC}$ at $F$ and $G$, respectively. What is the area of quadrilateral $FDBG$?

$\textbf{(A) }60 \qquad \textbf{(B) }65 \qquad \textbf{(C) }70 \qquad \textbf{(D) }75 \qquad \textbf{(E) }80 \qquad$

## hardness of problem

The hardness of this problem is average medium or on a scale of 10, a 5. The problem needs no extension or addition of lines, as all you need to know is the angle bisector theorem and how to calculate the area of divided portions.

~justin6688

## Solution 1

Let $BC = a$, $BG = x$, $GC = y$, and the length of the perpendicular to $BC$ through $A$ be $h$. By angle bisector theorem, we have that $$\frac{50}{x} = \frac{10}{y},$$ where $y = -x+a$. Therefore substituting we have that $BG=\frac{5a}{6}$. By similar triangles, we have that $DF=\frac{5a}{12}$, and the height of this trapezoid is $\frac{h}{2}$. Then, we have that $\frac{ah}{2}=120$. We wish to compute $\frac{5a}{8}\cdot\frac{h}{2}$, and we have that it is $\boxed{75}$ by substituting.

## Solution 2

For this problem, we have $\triangle{ADE}\sim\triangle{ABC}$ because of SAS and $DE = \frac{BC}{2}$. Therefore, $\bigtriangleup ADE$ is a quarter of the area of $\bigtriangleup ABC$, which is $30$. Subsequently, we can compute the area of quadrilateral $BDEC$ to be $120 - 30 = 90$. Using the angle bisector theorem in the same fashion as the previous problem, we get that $\overline{BG}$ is $5$ times the length of $\overline{GC}$. We want the larger piece, as described by the problem. Because the heights are identical, one area is $5$ times the other, and $\frac{5}{6} \cdot 90 = \boxed{75}$.

## Solution 3

The area of $\bigtriangleup ABG$ to the area of $\bigtriangleup ACG$ is $5:1$ by Law of Sines. So the area of $\bigtriangleup ABG$ is $100$. Since $\overline{DE}$ is the midsegment of $\bigtriangleup ABC$, so $\overline{DF}$ is the midsegment of $\bigtriangleup ABG$ . So the area of $\bigtriangleup ADF$ to the area of $\bigtriangleup ABG$ is $1:4$ , so the area of $\bigtriangleup ACG$ is $25$, by similar triangles. Therefore the area of quad $FDBG$ is $100-25=\boxed{75}$

## Solution 4

The area of quadrilateral $FDBG$ is the area of $\bigtriangleup ABG$ minus the area of $\bigtriangleup ADF$. Notice, $\overline{DE} || \overline{BC}$, so $\bigtriangleup ABG \sim \bigtriangleup ADF$, and since $\overline{AD}:\overline{AB}=1:2$, the area of $\bigtriangleup ADF:\bigtriangleup ABG=(1:2)^2=1:4$. Given that the area of $\bigtriangleup ABC$ is $120$, using $\frac{bh}{2}$ on side $AB$ yields $\frac{50h}{2}=120\implies h=\frac{240}{50}=\frac{24}{5}$. Using the Angle Bisector Theorem, $\overline{BG}:\overline{BC}=50:(10+50)=5:6$, so the height of $\bigtriangleup ABG: \bigtriangleup ACB=5:6$. Therefore our answer is $\big[ FDBG\big] = \big[ABG\big]-\big[ ADF\big] = \big[ ABG\big]\big(1-\frac{1}{4}\big)=\frac{3}{4}\cdot \frac{bh}{2}=\frac{3}{8}\cdot 50\cdot \frac{5}{6}\cdot \frac{24}{5}=\frac{3}{8}\cdot 200=\boxed{75}$

## Solution 5: Trig

We try to find the area of quadrilateral $FDBG$ by subtracting the area outside the quadrilateral but inside triangle $ABC$. Note that the area of $\triangle ADE$ is equal to $\frac{1}{2} \cdot 25 \cdot 5 \cdot \sin{A}$ and the area of triangle $ABC$ is equal to $\frac{1}{2} \cdot 50 \cdot 10 \cdot \sin A$. The ratio $\frac{[ADE]}{[ABC]}$ is thus equal to $\frac{1}{4}$ and the area of triangle $ADE$ is $\frac{1}{4} \cdot 120 = 30$. Let side $BC$ be equal to $6x$, then $BG = 5x, GC = x$ by the angle bisector theorem. Similarly, we find the area of triangle $AGC$ to be $\frac{1}{2} \cdot 10 \cdot x \cdot \sin C$ and the area of triangle $ABC$ to be $\frac{1}{2} \cdot 6x \cdot 10 \cdot \sin C$. A ratio between these two triangles yields $\frac{[ACG]}{[ABC]} = \frac{x}{6x} = \frac{1}{6}$, so $[AGC] = 20$. Now we just need to find the area of triangle $AFE$ and subtract it from the combined areas of $[ADE]$ and $[ACG]$, since we count it twice. Note that the angle bisector theorem also applies for $\triangle ADE$ and $\frac{AE}{AD} = \frac{1}{5}$, so thus $\frac{EF}{ED} = \frac{1}{6}$ and we find $[AFE] = \frac{1}{6} \cdot 30 = 5$, and the area outside $FDBG$ must be $[ADE] + [AGC] - [AFE] = 30 + 20 - 5 = 45$, and we finally find $[FDBG] = [ABC] - 45 = 120 -45 = \boxed{75}$, and we are done.

## Solution 6: Areas

$[asy] draw((0,0)--(1,3)--(5,0)--cycle); draw((0,0)--(2,2.25)); draw((0.5,1.5)--(2.5,0)); label("A",(0,0),SW); label("B",(5,0),SE); label("C",(1,3),N); label("G",(2,2.25),NE); label("D",(2.5,0),S); label("E",(0.5,1.5),NW); label("3Y",(2.5,0.75),N); label("Y",(1,0.2),N); label("X",(0.5,0.5),N); label("3X",(1.25,1.75),N); [/asy]$ Give triangle $AEF$ area X. Then, by similarity, since $\frac{AC}{AE} = \frac{2}{1}$, $ACG$ has area 4X. Thus, $FGCE$ has area 3X. Doing the same for triangle $AGB$, we get that triangle $AFD$ has area Y and quadrilateral $GFDB$ has area 3Y. Since $AEF$ has the same height as $AFD$, the ratios of the areas is equal to the ratios of the bases. Because of the Angle Bisector Theorem, $\frac{CG}{GB} = \frac{1}{5}$. So, $\frac{[AEF]}{[AFD]} = \frac{1}{5}$. Since $AEF$ has area X, we can write the equation 5X = Y and substitute 5X for Y. $[asy] draw((0,0)--(1,3)--(5,0)--cycle); draw((0,0)--(2,2.25)); draw((0.5,1.5)--(2.5,0)); label("A",(0,0),SW); label("B",(5,0),SE); label("C",(1,3),N); label("G",(2,2.25),NE); label("D",(2.5,0),S); label("E",(0.5,1.5),NW); label("",(2.5,0.75),N); label("",(1,0.2),N); label("F", (1, 1.5), N); label("",(0.5,1.5),N); label("",(1.25,1.75),N); [/asy]$ Now we can solve for X by adding up all the sums. X + 3X + 5X + 15X = 120, so X = 5. Since we want to find $GFDB$, we substitute 5 for 15X to get $\boxed{75}$. $\sim$krishkhushi09

~ dolphin7

~ pi_is_3.14