Difference between revisions of "2016 AMC 12A Problems/Problem 12"

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Problem 12

In $\triangle ABC$ , $AB = 6$ , $BC = 7$ , and $CA = 8$ . Point $D$ lies on $\overline{BC}$ , and $\overline{AD}$ bisects $\angle BAC$ . Point $E$ lies on $\overline{AC}$ , and $\overline{BE}$ bisects $\angle ABC$ . The bisectors intersect at $F$ . What is the ratio $AF$ : $FD$ ?

$[asy] pair A = (0,0), B=(6,0), C=intersectionpoints(Circle(A,8),Circle(B,7))[0], F=incenter(A,B,C), D=extension(A,F,B,C),E=extension(B,F,A,C); draw(A--B--C--A--D^^B--E); label("$A$",A,SW); label("$B$",B,SE); label("$C$",C,N); label("$D$",D,NE); label("$E$",E,NW); label("$F$",F,1.5*N); [/asy]$

$\textbf{(A)}\ 3:2\qquad\textbf{(B)}\ 5:3\qquad\textbf{(C)}\ 2:1\qquad\textbf{(D)}\ 7:3\qquad\textbf{(E)}\ 5:2$

Solution 1

Simply notice that AD and BE intersect at the triangle's centroid, as they bisect angle CAB and angle ABC respectively. Therefore, due to the properties of a centroid, we know that the ratio of AF to FD is 2:1. Answer: C ~the_noob_moment

Solution 2

By the angle bisector theorem, $\frac{AB}{AE} = \frac{CB}{CE}$

$\frac{6}{AE} = \frac{7}{8 - AE}$ so $AE = \frac{48}{13}$

Similarly, $CD = 4$ .

Now, we use mass points. Assign point $C$ a mass of $1$ .

$mC \cdot CD = mB \cdot DB$ , so $mB = \frac{4}{3}$

Similarly, $A$ will have a mass of $\frac{7}{6}$

$mD = mC + mB = 1 + \frac{4}{3} = \frac{7}{3}$

So $\frac{AF}{AD} = \frac{mD}{mA} = \boxed{\textbf{(C)}\; 2 : 1}$

Solution 3

Denote $[\triangle{ABC}]$ as the area of triangle ABC and let $r$ be the inradius. Also, as above, use the angle bisector theorem to find that $BD = 3$ . There are two ways to continue from here:

$1.$ Note that $F$ is the incenter. Then, $\frac{AF}{FD} = \frac{[\triangle{AFB}]}{[\triangle{BFD}]} = \frac{AB * \frac{r}{2}}{BD * \frac{r}{2}} = \frac{AB}{BD} = \boxed{\textbf{(C)}\; 2 : 1}$

$2.$ Apply the angle bisector theorem on $\triangle{ABD}$ to get $\frac{AF}{FD} = \frac{AB}{BD} = \frac{6}{3} = \boxed{\textbf{(C)}\; 2 : 1}$

Solution 4

Draw the third angle bisector, and denote the point where this bisector intersects $AB$ as $P$ . Using angle bisector theorem, we see $AE=48/13 , EC=56/13, AP=16/5, PB=14/5$ . Applying Van Aubel's Theorem, $AF/FD=(48/13)/(56/13) + (16/5)/(14/5)=(6/7)+(8/7)=14/7=2/1$ , and so the answer is $\boxed{\textbf{(C)}\; 2 : 1}$ .

@@ Line 9: / Line 9: @@
 == Solution 1==
-Applying the angle bisector theorem to <math>\triangle ABC</math> with <math>\angle CAB</math> being bisected by <math>AD</math>, we have
+Simply notice that AD and BE intersect at the triangle's centroid, as they bisect angle CAB and angle ABC respectively. Therefore, due to the properties of a centroid, we know that the ratio of AF to FD is 2:1. Answer: C  ~the_noob_moment
-<cmath>\frac{CD}{AC}=\frac{BD}{AB}.</cmath>
-Thus, we have
-<cmath>\frac{CD}{8}=\frac{BD}{6},</cmath>
-and cross multiplying and dividing by <math>2</math> gives us
-<cmath>3\cdot CD=4\cdot BD.</cmath>
-Since <math>CD+BD=BC=7</math>, we can substitute <math>CD=7-BD</math> into the former equation. Therefore, we get <math>3(7-BD)=4BD</math>, so <math>BD=3</math>.
-Apply the angle bisector theorem again to <math>\triangle ABD</math> with <math>\angle ABC</math> being bisected.  This gives us
-<cmath>\frac{AB}{AF}=\frac{BD}{FD},</cmath>
-and since <math>AB=6</math> and <math>BD=3</math>, we have
-<cmath>\frac{6}{AF}=\frac{3}{FD}.</cmath>
-Cross multiplying and dividing by <math>3</math> gives us
-<cmath>AF=2\cdot FD,</cmath>
-and dividing by <math>FD</math> gives us
-<cmath>\frac{AF}{FD}=\frac{2}{1}.</cmath>
-Therefore,
-<cmath>AF:FD=\frac{AF}{FD}=\frac{2}{1}=\boxed{\textbf{(C)}\; 2 : 1}.</cmath>
 == Solution 2==

2016 AMC 12A (Problems • Answer Key • Resources)
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