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2016 AMC 12A Problems/Problem 12: Difference between revisions

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== Solution 4==
== Solution 4==
One only needs the angle bisector theorem, the segment addition postulate, and some simple algebra to solve this question.
One only needs the angle bisector theorem to solve this question.


The question asks for AF:DF. Apply the angle bisector theorem to <math>\triangle ABD</math> to get the ratio <math>\frac {AF}{DF}</math> :
The question asks for <math>AF:FD = \frac{AF}{FD}</math>. Apply the angle bisector theorem to <math>\triangle BAD</math> to get
<cmath>\frac{AF}{FD} = \frac{AB}{BD}.</cmath>


<math>\frac {AF}{AB}</math> = <math>\frac {DF}{BD}</math> or, equivalently,
<math>AB = 6</math> is given. To find <math>BD</math>, apply the angle bisector theorem to <math>\triangle ABC</math> to get
<cmath>\frac{BD}{DC} = \frac{BA}{AC} = \frac{6}{8} = \frac{3}{4}.</cmath>


<math>\frac {AF}{DF}</math> = <math>\frac {AB}{BD}</math>.  
Since
<cmath>BD + DC = BC = 7,</cmath>
it is immediately obvious that <math>BD = 3</math>, <math>DC = 4</math> satisfies both equations.


AB is given. To find BD apply the angle bisector theorem to <math>\triangle ABC</math> to get:
Thus,
 
<cmath>AF:FD = AB:BD = 6:3 = \boxed{\textbf{(C)}\ 2:1}.</cmath>
<math>\frac {BD}{AB}</math> = <math>\frac {CD}{AC}</math>
~revision by [[User:emerald_block|emerald_block]]
 
---> <math>\frac {BD}{AB}</math> = <math>\frac {BC - BD}{AC}</math> since, by the segment addition postulate, BD + CD = BC
 
---> BD = <math>\frac {AB*BC}{AC + AB}</math>.
 
Substituting this expression for BD into the proportion <math>\frac {AF}{DF}</math> = <math>\frac {AB}{BD}</math> yields:
 
<math>\frac {AF}{DF}</math> = <math>\frac {AB}{BD}</math> = <math>\frac {AC + AB}{BC}</math> = <math>\frac {8 + 6}{7}</math> = 2.


==See Also==
==See Also==
{{AMC12 box|year=2016|ab=A|num-b=11|num-a=13}}
{{AMC12 box|year=2016|ab=A|num-b=11|num-a=13}}
{{MAA Notice}}
{{MAA Notice}}

Revision as of 11:08, 3 November 2021

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

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

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

Similarly, $CD = 4$.

There are two ways to solve from here. First way:

Note that $DB = 7 - 4 = 3.$ By the angle bisector theorem on $\triangle ADB,$ $\frac{AF}{FD} = \frac{AB}{DB} = \frac{6}{3}.$ Thus the answer is $\boxed{\textbf{(C)}\; 2 : 1}$

Second way:

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

Solution 2

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 3

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 $\boxed{\textbf{(C)}\; 2 : 1}$.

Solution 4

One only needs the angle bisector theorem to solve this question.

The question asks for $AF:FD = \frac{AF}{FD}$. Apply the angle bisector theorem to $\triangle BAD$ to get \[\frac{AF}{FD} = \frac{AB}{BD}.\]

$AB = 6$ is given. To find $BD$, apply the angle bisector theorem to $\triangle ABC$ to get \[\frac{BD}{DC} = \frac{BA}{AC} = \frac{6}{8} = \frac{3}{4}.\]

Since \[BD + DC = BC = 7,\] it is immediately obvious that $BD = 3$, $DC = 4$ satisfies both equations.

Thus, \[AF:FD = AB:BD = 6:3 = \boxed{\textbf{(C)}\ 2:1}.\] ~revision by emerald_block

See Also

2016 AMC 12A (Problems • Answer Key • Resources)
Preceded by
Problem 11 		Followed by
Problem 13
1 • 2 • 3 • 4 • 5 • 6 • 7 • 8 • 9 • 10 • 11 • 12 • 13 • 14 • 15 • 16 • 17 • 18 • 19 • 20 • 21 • 22 • 23 • 24 • 25
All AMC 12 Problems and Solutions

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