Difference between revisions of "2011 AMC 10B Problems/Problem 9"

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== Problem==
 
== Problem==
  
The area of <math>\triangle</math><math>EBD</math> is one third of the area of <math>3-4-5</math> <math>\triangle</math><math>ABC</math>. Segment <math>DE</math> is perpendicular to segment <math>AB</math>. What is <math>BD</math>? <p>
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The area of <math>\triangle</math><math>EBD</math> is one third of the area of <math>\triangle</math><math>ABC</math>. Segment <math>DE</math> is perpendicular to segment <math>AB</math>. What is <math>BD</math>? <p>
 
<center><asy>
 
<center><asy>
 
unitsize(10mm);
 
unitsize(10mm);
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[[Category: Introductory Geometry Problems]]
 
[[Category: Introductory Geometry Problems]]
  
== Solution ==
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== Solution 1==
  
 
<math>\triangle ABC \sim \triangle EBD</math> by AA Similarity. Therefore <math>DE = \frac{3}{4} BD</math>. Find the areas of the triangles.
 
<math>\triangle ABC \sim \triangle EBD</math> by AA Similarity. Therefore <math>DE = \frac{3}{4} BD</math>. Find the areas of the triangles.
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\end{align*}
 
\end{align*}
 
</cmath>
 
</cmath>
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== Solution 2 ==
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<math>\triangle ABC \sim \triangle EBD</math> by AA Similarity. Since the area of <math>\triangle EBD</math> is <math>\frac{1}{3}</math> of <math>\triangle ABC</math> and the bases/heights are in the same ratio, we use the formula forarea of a triangle for these ratios. Thus,
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<cmath>\frac{1}{3}[\triangle ABC] = [\triangle EBD]</cmath>
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<cmath>\frac{1}{3} \times \frac{1}{2} \times AC \times BC = \frac{1}{2} \times ED \times DB</cmath>
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In order to scale the sides of ED and DB to make <math>\frac{1}{3}</math> (since the ratios of sides are the same), we take the square root of <math>\frac{1}{3} = \frac{\sqrt(3)}{3}</math> to scale each side by the same amount.
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Thus <math>BD = 4 \times \frac{\sqrt(3)}{3}</math> and the answer is <math>BD = \boxed{\textbf{(D)} \frac{4\sqrt{3}}{3}}</math>
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==Solution 3 (Shortcut)==
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The ratio of the areas of <math>\triangle</math><math>EBD</math> and <math>\triangle</math><math>ABC</math> is <math>1 : 3</math>, meaning the ratio of the sides is <math>1 : \sqrt{3}</math>. The only answer choice involving <math>\sqrt{3}</math> is <math>\boxed{\textbf{(D) } \frac{4\sqrt{3}}{3}}</math>.
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-Solution by Joeya
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==Remark (slightly more vigorous than Solution 3)==
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The ratio of the areas is equal to twice the ratio of sides (in similar figures) because area is a second-degree property of similar figures. So like solution 3, the ratio of sides is <math>\sqrt{\frac{1}{3}}=\frac{1}{\sqrt{3}}=\sqrt{3}.</math> By similarity, <math>\frac{ED}{DB}=\frac{\sqrt{3}}{DB}=\frac{3}{4}</math>, so solving for DB, we get <math>\boxed{\textbf{(D) } \frac{4\sqrt{3}}{3}}</math>.
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~JH. L
  
 
== See Also==
 
== See Also==

Revision as of 05:38, 25 June 2022

Problem

The area of $\triangle$$EBD$ is one third of the area of $\triangle$$ABC$. Segment $DE$ is perpendicular to segment $AB$. What is $BD$?

[asy] unitsize(10mm); defaultpen(linewidth(.8pt)+fontsize(10pt)); dotfactor=4;  pair A=(0,0), B=(5,0), C=(1.8,2.4), D=(5-4sqrt(3)/3,0), E=(5-4sqrt(3)/3,sqrt(3)); pair[] ps={A,B,C,D,E};  draw(A--B--C--cycle); draw(E--D); draw(rightanglemark(E,D,B));  dot(ps); label("$A$",A,SW); label("$B$",B,SE); label("$C$",C,N); label("$D$",D,S); label("$E$",E,NE); label("$3$",midpoint(A--C),NW); label("$4$",midpoint(C--B),NE); label("$5$",midpoint(A--B),SW); [/asy]

$\textbf{(A)}\ \frac{4}{3} \qquad\textbf{(B)}\ \sqrt{5} \qquad\textbf{(C)}\ \frac{9}{4} \qquad\textbf{(D)}\ \frac{4\sqrt{3}}{3} \qquad\textbf{(E)}\ \frac{5}{2}$

Solution 1

$\triangle ABC \sim \triangle EBD$ by AA Similarity. Therefore $DE = \frac{3}{4} BD$. Find the areas of the triangles. \[\triangle ABC: 3 \times 4 \times \frac{1}{2} = 6\] \[\triangle EBD: BD \times \frac{3}{4} BD \times \frac{1}{2} = \frac{3}{8} BD ^2\] The area of $\triangle EBD$ is one third of the area of $\triangle ABC$. \begin{align*} \frac{3}{8} BD^2 &= 6 \times \frac{1}{3}\\ 9BD^2 &= 48\\ BD^2 &= \frac{16}{3}\\ BD &= \boxed{\textbf{(D)} \frac{4\sqrt{3}}{3}} \end{align*}

Solution 2

$\triangle ABC \sim \triangle EBD$ by AA Similarity. Since the area of $\triangle EBD$ is $\frac{1}{3}$ of $\triangle ABC$ and the bases/heights are in the same ratio, we use the formula forarea of a triangle for these ratios. Thus, \[\frac{1}{3}[\triangle ABC] = [\triangle EBD]\] \[\frac{1}{3} \times \frac{1}{2} \times AC \times BC = \frac{1}{2} \times ED \times DB\] In order to scale the sides of ED and DB to make $\frac{1}{3}$ (since the ratios of sides are the same), we take the square root of $\frac{1}{3} = \frac{\sqrt(3)}{3}$ to scale each side by the same amount.

Thus $BD = 4 \times \frac{\sqrt(3)}{3}$ and the answer is $BD = \boxed{\textbf{(D)} \frac{4\sqrt{3}}{3}}$

Solution 3 (Shortcut)

The ratio of the areas of $\triangle$$EBD$ and $\triangle$$ABC$ is $1 : 3$, meaning the ratio of the sides is $1 : \sqrt{3}$. The only answer choice involving $\sqrt{3}$ is $\boxed{\textbf{(D) } \frac{4\sqrt{3}}{3}}$.

-Solution by Joeya


Remark (slightly more vigorous than Solution 3)

The ratio of the areas is equal to twice the ratio of sides (in similar figures) because area is a second-degree property of similar figures. So like solution 3, the ratio of sides is $\sqrt{\frac{1}{3}}=\frac{1}{\sqrt{3}}=\sqrt{3}.$ By similarity, $\frac{ED}{DB}=\frac{\sqrt{3}}{DB}=\frac{3}{4}$, so solving for DB, we get $\boxed{\textbf{(D) } \frac{4\sqrt{3}}{3}}$.

~JH. L

See Also

2011 AMC 10B (ProblemsAnswer KeyResources)
Preceded by
Problem 8
Followed by
Problem 10
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 10 Problems and Solutions

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