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Difference between revisions of "2002 AMC 10A Problems/Problem 25"

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== Problem ==
 
== Problem ==
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In [[trapezoid]] <math>ABCD</math> with bases <math>AB</math> and <math>CD</math>, we have <math>AB = 52</math>, <math>BC = 12</math>, <math>CD = 39</math>, and <math>DA = 5</math>. The area of <math>ABCD</math> is
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<asy>
 
<asy>
 
pair A,B,C,D;
 
pair A,B,C,D;
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label("5",(D+A)/2,W);
 
label("5",(D+A)/2,W);
 
</asy>
 
</asy>
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In [[trapezoid]] <math>ABCD</math> with bases <math>AB</math> and <math>CD</math>, we have <math>AB = 52</math>, <math>BC = 12</math>, <math>CD = 39</math>, and <math>DA = 5</math>. The area of <math>ABCD</math> is
 
  
 
<math>\text{(A)}\ 182 \qquad \text{(B)}\ 195 \qquad \text{(C)}\ 210 \qquad \text{(D)}\ 234 \qquad \text{(E)}\ 260</math>
 
<math>\text{(A)}\ 182 \qquad \text{(B)}\ 195 \qquad \text{(C)}\ 210 \qquad \text{(D)}\ 234 \qquad \text{(E)}\ 260</math>
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By continuing to split <math>\overline{AB}</math> and <math>\overline{CD}</math> into segments of length <math>13,</math> we can connect these vertices in a "zig-zag," creating seven congruent right triangles, each with sides <math>5,12,</math> and <math>13,</math> and each with area <math>30.</math> The total area is therefore <math>7 \cdot 30 = \boxed{\textbf{(C)} 210}.</math>
 
By continuing to split <math>\overline{AB}</math> and <math>\overline{CD}</math> into segments of length <math>13,</math> we can connect these vertices in a "zig-zag," creating seven congruent right triangles, each with sides <math>5,12,</math> and <math>13,</math> and each with area <math>30.</math> The total area is therefore <math>7 \cdot 30 = \boxed{\textbf{(C)} 210}.</math>
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Alternative: Instead of creating seven congruent right triangles, we can find the height of parallelogram <math>AECD</math> by drawing an altitude from <math>D</math> to side <math>AE</math>, creating the new point <math>F</math>. By recognizing that triangle <math>DAF</math> is similar to triangle <math>BEC</math>, we can use properties of similar triangles and find that <math>DE = 12 \cdot \frac{5}{13} = \frac{60}{13}</math>. Thus, the area of parallelogram <math>AECD</math> is <math>\frac{60}{13} \cdot 39 = 180</math>. Finally, we add the areas of the parallelogram <math>AECD</math> and the right triangle <math>BEC</math> together and we get <math>180 + 30 = \boxed{\textbf{(C)} 210}</math>. ~scarletsyc
  
 
=== Solution 2 but quicker ===
 
=== Solution 2 but quicker ===
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First note how the answer choices are all integers.
 
First note how the answer choices are all integers.
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The area of the trapezoid is <math>\frac{39+52}{2} * h = \frac{91}{2} h</math>. So h divides 2. Let <math>x</math> be <math>2h</math>. The area is now <math>91x</math>.  
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The area of the trapezoid is <math>\frac{39+52}{2} \cdot h = \frac{91}{2} h</math>. So h divides 2. Let <math>x</math> be <math>2h</math>. The area is now <math>91x</math>.  
 
Trying <math>x=1</math> and <math>x=2</math> can easily be seen to not work. Those make the only integers possible so now you know x is a fraction.  
 
Trying <math>x=1</math> and <math>x=2</math> can easily be seen to not work. Those make the only integers possible so now you know x is a fraction.  
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Since the area is an integer the denominator of x must divide either 13 or 7 since <math>91 = 13*7</math>.
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Since the area is an integer the denominator of x must divide either 13 or 7 since <math>91 = 13\cdot7</math>.
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Seeing how <math>39 = 3*13</math> and <math>52 = 4*13</math> assume that the denominator divides 13. Letting <math>y = \frac{x}{13}</math> the area is now <math>7y</math>.
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Seeing how <math>39 = 3\cdot13</math> and <math>52 = 4\cdot13</math> assume that the denominator divides 13. Letting <math>y = \frac{x}{13}</math> the area is now <math>7y</math>.
 
Note that (A) and (C) are the only multiples of 7. We know that A doesn't work because that would mean <math>h</math> is <math>4</math> which we ruled out.  
 
Note that (A) and (C) are the only multiples of 7. We know that A doesn't work because that would mean <math>h</math> is <math>4</math> which we ruled out.  
 
So the answer is <math>\boxed{\textbf{(C)} 210}</math>. - megateleportingrobots
 
So the answer is <math>\boxed{\textbf{(C)} 210}</math>. - megateleportingrobots

Latest revision as of 06:05, 19 December 2022

Problem

In trapezoid $ABCD$ with bases $AB$ and $CD$, we have $AB = 52$, $BC = 12$, $CD = 39$, and $DA = 5$. The area of $ABCD$ is

[asy] pair A,B,C,D; A=(0,0); B=(52,0); C=(38,20); D=(5,20); dot(A); dot(B); dot(C); dot(D); draw(A--B--C--D--cycle); label("$A$",A,S); label("$B$",B,S); label("$C$",C,N); label("$D$",D,N); label("52",(A+B)/2,S); label("39",(C+D)/2,N); label("12",(B+C)/2,E); label("5",(D+A)/2,W); [/asy]

$\text{(A)}\ 182 \qquad \text{(B)}\ 195 \qquad \text{(C)}\ 210 \qquad \text{(D)}\ 234 \qquad \text{(E)}\ 260$

Solution

Solution 1

It shouldn't be hard to use trigonometry to bash this and find the height, but there is a much easier way. Extend $\overline{AD}$ and $\overline{BC}$ to meet at point $E$:

[asy] size(250); defaultpen(0.8); pair A=(0,0), B = (52,0), C=(52-144/13,60/13), D=(25/13,60/13), F=(100/13,240/13); draw(A--B--C--D--cycle); draw(D--F--C,dashed); label("\(A\)",A,S); label("\(B\)",B,S); label("\(C\)",C,NE); label("\(D\)",D,W); label("\(E\)",F,N); label("39",(C+D)/2,N); label("52",(A+B)/2,S); label("5",(A+D)/2,E); label("12",(B+C)/2,WSW); [/asy]

Since $\overline{AB} || \overline{CD}$ we have $\triangle AEB \sim \triangle DEC$, with the ratio of proportionality being $\frac {39}{52} = \frac {3}{4}$. Thus \begin{align*} \frac {CE}{CE + 12} = \frac {3}{4} & \Longrightarrow CE = 36 \\ \frac {DE}{DE + 5} = \frac {3}{4} & \Longrightarrow DE = 15 \end{align*} So the sides of $\triangle CDE$ are $15,36,39$, which we recognize to be a $5 - 12 - 13$ right triangle. Therefore (we could simplify some of the calculation using that the ratio of areas is equal to the ratio of the sides squared), \[[ABCD] = [ABE] - [CDE] = \frac {1}{2}\cdot 20 \cdot 48 - \frac {1}{2} \cdot 15 \cdot 36 = \boxed{\mathrm{(C)}\ 210}\]

Solution 2

Draw altitudes from points $C$ and $D$:

[asy] unitsize(0.2cm); defaultpen(0.8); pair A=(0,0), B = (52,0), C=(52-144/13,60/13), D=(25/13,60/13), E=(52-144/13,0), F=(25/13,0); draw(A--B--C--D--cycle); draw(C--E,dashed); draw(D--F,dashed); label("\(A\)",A,SW); label("\(B\)",B,S); label("\(C\)",C,NE); label("\(D\)",D,N); label("\(D'\)",F,SSE); label("\(C'\)",E,S); label("39",(C+D)/2,N); label("52",(A+B)/2,S); label("5",(A+D)/2,W); label("12",(B+C)/2,ENE); [/asy]

Translate the triangle $ADD'$ so that $DD'$ coincides with $CC'$. We get the following triangle:

[asy] unitsize(0.2cm); defaultpen(0.8); pair A=(0,0), B = (13,0), C=(25/13,60/13), F=(25/13,0); draw(A--B--C--cycle); draw(C--F,dashed); label("\(A'\)",A,SW); label("\(B\)",B,S); label("\(C\)",C,N); label("\(C'\)",F,SE); label("5",(A+C)/2,W); label("12",(B+C)/2,ENE); [/asy]

The length of $A'B$ in this triangle is equal to the length of the original $AB$, minus the length of $CD$. Thus $A'B = 52 - 39 = 13$.

Therefore $A'BC$ is a well-known $(5,12,13)$ right triangle. Its area is $[A'BC]=\frac{A'C\cdot BC}2 = \frac{5\cdot 12}2 = 30$, and therefore its altitude $CC'$ is $\frac{[A'BC]}{A'B} = \frac{60}{13}$.

Now the area of the original trapezoid is $\frac{(AB+CD)\cdot CC'}2 = \frac{91 \cdot 60}{13 \cdot 2} = 7\cdot 30 = \boxed{\mathrm{(C)}\ 210}$

Solution 3

Draw altitudes from points $C$ and $D$:

[asy] unitsize(0.2cm); defaultpen(0.8); pair A=(0,0), B = (52,0), C=(52-144/13,60/13), D=(25/13,60/13), E=(52-144/13,0), F=(25/13,0); draw(A--B--C--D--cycle); draw(C--E,dashed); draw(D--F,dashed); label("\(A\)",A,SW); label("\(B\)",B,S); label("\(C\)",C,NE); label("\(D\)",D,N); label("\(D'\)",F,SSE); label("\(C'\)",E,S); label("39",(C+D)/2,N); label("52",(A+B)/2,S); label("5",(A+D)/2,W); label("12",(B+C)/2,ENE); [/asy]

Call the length of $AD'$ to be $y$, the length of $BC'$ to be $z$, and the height of the trapezoid to be $x$. By the Pythagorean Theorem, we have: \[z^2 + x^2 = 144\] \[y^2 + x^2 = 25\]

Subtracting these two equation yields: \[z^2-y^2=119 \implies (z+y)(z-y)=119\]

We also have: $z+y=52-39=13$.

We can substitute the value of $z+y$ into the equation we just obtained, so we now have:

\[(13) (z-y)=119 \implies z-y=\frac{119}{13}\].

We can add the $z+y$ and the $z-y$ equation to find the value of $z$, which simplifies down to be $\frac{144}{13}$. Finally, we can plug in $z$ and use the Pythagorean theorem to find the height of the trapezoid.

\[\frac{12^4}{13^2} + x^2 = 12^2 \implies x^2 = \frac{(12^2)(13^2)}{13^2} -\frac{12^4}{13^2} \implies x^2 = \frac{(12 \cdot 13)^2 - (144)^2}{13^2} \implies x^2 = \frac{(156+144)(156-144)}{13^2} \implies x = \sqrt{\frac{3600}{169}} = \frac{60}{13}\]

Now that we have the height of the trapezoid, we can multiply this by the median to find our answer.

The median of the trapezoid is $\frac{39+52}{2} = \frac{91}{2}$, and multiplying this and the height of the trapezoid gets us:

\[\frac{60 \cdot 91}{13 \cdot 2} = \boxed{\mathrm{(C)}\ 210}\]

Solution 4

We construct a line segment parallel to $\overline{AD}$ from point $C$ to line $\overline{AB},$ and label the intersection of this segment with line $\overline{AB}$ as point $E.$ Then quadrilateral $AECD$ is a parallelogram, so $CE=5, AE=39,$ and $EB=13.$ Triangle $EBC$ is therefore a right triangle, with area $\frac12 \cdot 5 \cdot 12 = 30.$

By continuing to split $\overline{AB}$ and $\overline{CD}$ into segments of length $13,$ we can connect these vertices in a "zig-zag," creating seven congruent right triangles, each with sides $5,12,$ and $13,$ and each with area $30.$ The total area is therefore $7 \cdot 30 = \boxed{\textbf{(C)} 210}.$

Alternative: Instead of creating seven congruent right triangles, we can find the height of parallelogram $AECD$ by drawing an altitude from $D$ to side $AE$, creating the new point $F$. By recognizing that triangle $DAF$ is similar to triangle $BEC$, we can use properties of similar triangles and find that $DE = 12 \cdot \frac{5}{13} = \frac{60}{13}$. Thus, the area of parallelogram $AECD$ is $\frac{60}{13} \cdot 39 = 180$. Finally, we add the areas of the parallelogram $AECD$ and the right triangle $BEC$ together and we get $180 + 30 = \boxed{\textbf{(C)} 210}$. ~scarletsyc

Solution 2 but quicker

From Solution $2$ we know that the the altitude of the trapezoid is $\frac{60}{13}$ and the triangle's area is $30$. Note that once we remove the triangle we get a rectangle with length $39$ and height $\frac{60}{13}$. The numbers multiply nicely to get $180+30=\boxed{(C) 210}$ -harsha12345

Quick Time Trouble Solution 5

First note how the answer choices are all integers. The area of the trapezoid is $\frac{39+52}{2} \cdot h = \frac{91}{2} h$. So h divides 2. Let $x$ be $2h$. The area is now $91x$. Trying $x=1$ and $x=2$ can easily be seen to not work. Those make the only integers possible so now you know x is a fraction. Since the area is an integer the denominator of x must divide either 13 or 7 since $91 = 13\cdot7$. Seeing how $39 = 3\cdot13$ and $52 = 4\cdot13$ assume that the denominator divides 13. Letting $y = \frac{x}{13}$ the area is now $7y$. Note that (A) and (C) are the only multiples of 7. We know that A doesn't work because that would mean $h$ is $4$ which we ruled out. So the answer is $\boxed{\textbf{(C)} 210}$. - megateleportingrobots

See also

2002 AMC 10A (Problems • Answer Key • Resources)
Preceded by
Problem 24 		Followed by
-(Last question)
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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