Difference between revisions of "2003 AMC 12A Problems/Problem 17"

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Square <math>ABCD</math> has sides of length <math>4</math>, and <math>M</math> is the midpoint of <math>\overline{CD}</math>. A circle with radius <math>2</math> and center <math>M</math> intersects a circle with radius <math>4</math> and center <math>A</math> at points <math>P</math> and <math>D</math>. What is the distance from <math>P</math> to <math>\overline{AD}</math>?
 
Square <math>ABCD</math> has sides of length <math>4</math>, and <math>M</math> is the midpoint of <math>\overline{CD}</math>. A circle with radius <math>2</math> and center <math>M</math> intersects a circle with radius <math>4</math> and center <math>A</math> at points <math>P</math> and <math>D</math>. What is the distance from <math>P</math> to <math>\overline{AD}</math>?
  
[[Image:5d50417537c6cddfb70810403c62787b889cdcb1.png]]
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<asy>
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pair A,B,C,D,M,P;
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D=(0,0);
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C=(10,0);
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B=(10,10);
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A=(0,10);
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M=(5,0);
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P=(8,4);
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dot(M);
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dot(P);
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draw(A--B--C--D--cycle,linewidth(0.7));
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draw((5,5)..D--C..cycle,linewidth(0.7));
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draw((7.07,2.93)..B--A--D..cycle,linewidth(0.7));
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label("$A$",A,NW);
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label("$B$",B,NE);
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label("$C$",C,SE);
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label("$D$",D,SW);
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label("$M$",M,S);
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label("$P$",P,N);
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</asy>
  
 
<math>\textbf{(A)}\ 3 \qquad \textbf{(B)}\ \frac {16}{5} \qquad \textbf{(C)}\ \frac {13}{4} \qquad \textbf{(D)}\ 2\sqrt {3} \qquad \textbf{(E)}\ \frac {7}{2}</math>
 
<math>\textbf{(A)}\ 3 \qquad \textbf{(B)}\ \frac {16}{5} \qquad \textbf{(C)}\ \frac {13}{4} \qquad \textbf{(D)}\ 2\sqrt {3} \qquad \textbf{(E)}\ \frac {7}{2}</math>
  
== Solution 1==
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== Solutions ==
 +
=== Solution 1===
  
 
Let <math>D</math> be the origin. <math>A</math> is the point <math>(0,4)</math> and <math>M</math> is the point <math>(2,0)</math>. We are given the radius of the quarter circle and semicircle as <math>4</math> and <math>2</math>, respectively, so their equations, respectively, are:
 
Let <math>D</math> be the origin. <math>A</math> is the point <math>(0,4)</math> and <math>M</math> is the point <math>(2,0)</math>. We are given the radius of the quarter circle and semicircle as <math>4</math> and <math>2</math>, respectively, so their equations, respectively, are:
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Thus <math>y = 0</math> and <math>y = \frac{8}{5}</math> making <math>x = 0</math> and <math>x = \frac{16}{5}</math>.
 
Thus <math>y = 0</math> and <math>y = \frac{8}{5}</math> making <math>x = 0</math> and <math>x = \frac{16}{5}</math>.
  
The first value of <math>0</math> is obviously referring to the x-coordinate of the point where the circles intersect at the origin, <math>D</math>, so the second value must be referring to the x coordinate of <math>P</math>. Since <math>\overline{AD}</math> is the y-axis, the distance to it from <math>P</math> is the same as the x-value of the coordinate of <math>P</math>, so the distance from <math>P</math> to <math>\overline{AD}</math> is <math>\frac{16}{5} \Rightarrow B</math>
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The first value of <math>0</math> is obviously referring to the x-coordinate of the point where the circles intersect at the origin, <math>D</math>, so the second value must be referring to the x coordinate of <math>P</math>. Since <math>\overline{AD}</math> is the y-axis, the distance to it from <math>P</math> is the same as the x-value of the coordinate of <math>P</math>, so the distance from <math>P</math> to <math>\overline{AD}</math> is <math>\frac{16}{5} \Rightarrow \boxed{B}.</math>
  
==Solution 2==
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=== Solution 2 ===
 +
<math>APMD</math> obviously forms a kite. Let the intersection of the diagonals be <math>E</math>. <math>AE+EM=AM=2\sqrt{5}</math> Let <math>AE=x</math>. Then, <math>EM=2\sqrt{5}-x</math>.
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 +
 
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By Pythagorean Theorem, <math>DE^2=4^2-AE^2=2^2-EM^2</math>. Thus, <math>16-x^2=4-(2\sqrt{5}-x)^2</math>. Simplifying, <math>x=\frac{8}{\sqrt{5}}</math>. By Pythagoras again, <math>DE=\frac{4}{\sqrt{5}}</math>. Then, the area of <math>ADP</math> is <math>DE\cdot AE=\frac{32}{5}</math>.
 +
 
 +
 
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Using <math>4</math> instead as the base, we can drop a altitude from P. <math>\frac{32}{5}=\frac{bh}{2}</math>. <math>\frac{32}{5}=\frac{4h}{2}</math>. Thus, the horizontal distance is  <math>\frac{16}{5} \implies \boxed{\textbf{(B)}\frac{16}{5}}</math>
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 +
 
 +
 
 +
~BJHHar
 +
 
 +
=== Solution 3 ===
  
 
Note that <math>P</math> is merely a reflection of <math>D</math> over <math>AM</math>. Call the intersection of <math>AM</math> and <math>DP</math> <math>X</math>. Drop perpendiculars from <math>X</math> and <math>P</math> to <math>AD</math>, and denote their respective points of intersection by <math>J</math> and <math>K</math>. We then have <math>\triangle DXJ\sim\triangle DPK</math>, with a scale factor of 2. Thus, we can find <math>XJ</math> and double it to get our answer. With some analytical geometry, we find that <math>XJ=\frac{8}{5}</math>, implying that <math>PK=\frac{16}{5}</math>.
 
Note that <math>P</math> is merely a reflection of <math>D</math> over <math>AM</math>. Call the intersection of <math>AM</math> and <math>DP</math> <math>X</math>. Drop perpendiculars from <math>X</math> and <math>P</math> to <math>AD</math>, and denote their respective points of intersection by <math>J</math> and <math>K</math>. We then have <math>\triangle DXJ\sim\triangle DPK</math>, with a scale factor of 2. Thus, we can find <math>XJ</math> and double it to get our answer. With some analytical geometry, we find that <math>XJ=\frac{8}{5}</math>, implying that <math>PK=\frac{16}{5}</math>.
  
==Solution 3==
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=== Solution 4 ===
 
As in Solution 2, draw in <math>DP</math> and <math>AM</math> and denote their intersection point <math>X</math>. Next, drop a perpendicular from <math>P</math> to <math>AD</math> and denote the foot as <math>Z</math>. <math>AP \cong AD</math> as they are both radii and similarly <math>DM \cong MP</math> so <math>APMD</math> is a kite and <math>DX \perp XM</math> by a well-known theorem.  
 
As in Solution 2, draw in <math>DP</math> and <math>AM</math> and denote their intersection point <math>X</math>. Next, drop a perpendicular from <math>P</math> to <math>AD</math> and denote the foot as <math>Z</math>. <math>AP \cong AD</math> as they are both radii and similarly <math>DM \cong MP</math> so <math>APMD</math> is a kite and <math>DX \perp XM</math> by a well-known theorem.  
  
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Manipulating similar triangles gives us <math>PZ=\frac{16}{5}</math>
 
Manipulating similar triangles gives us <math>PZ=\frac{16}{5}</math>
  
==Solution 4==
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=== Solution 5 ===
 
Using the double-angle formula for sine, what we need to find is <math>AP\cdot \sin(DAP) = AP\cdot  2\sin( DAM) \cos(DAM) = 4\cdot 2\cdot \frac{2}{\sqrt{20}}\cdot\frac{4}{\sqrt{20}} = \frac{16}{5}</math>.
 
Using the double-angle formula for sine, what we need to find is <math>AP\cdot \sin(DAP) = AP\cdot  2\sin( DAM) \cos(DAM) = 4\cdot 2\cdot \frac{2}{\sqrt{20}}\cdot\frac{4}{\sqrt{20}} = \frac{16}{5}</math>.
 +
==Solution 6(LoC)==
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We use the Law of Cosines:
 +
 +
<math>32-32 \cos \theta = 8 + 8 \cos \theta </math>
 +
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<math>\frac{3}{5} = \cos \theta </math>
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<math>2 + 2*\frac{3}{5} = \frac{16}{5}</math>
 +
 +
=== Solution 7 ===
 +
 +
Let <math>H</math> be the foot of the perpendicular from <math>P</math> to <math>CD</math>, and let <math>HD = x</math>. Then we have <math>HC = 4-x</math>, and <math>PH = 4 - \sqrt{16 - x^2}</math>. Since <math>\triangle DHP \sim \triangle PHC</math>, we have <math>HP^2 = DH \cdot HC</math>, or <math>-x^2 + 4x = 16 - 8\sqrt{16-x^2}</math>. Solving gives <math>x = \frac{16}{5}</math>.
  
 
== See Also ==
 
== See Also ==
 
{{AMC12 box|year=2003|ab=A|num-b=16|num-a=18}}
 
{{AMC12 box|year=2003|ab=A|num-b=16|num-a=18}}
 
{{MAA Notice}}
 
{{MAA Notice}}

Latest revision as of 11:04, 31 August 2021

Problem

Square $ABCD$ has sides of length $4$, and $M$ is the midpoint of $\overline{CD}$. A circle with radius $2$ and center $M$ intersects a circle with radius $4$ and center $A$ at points $P$ and $D$. What is the distance from $P$ to $\overline{AD}$?

[asy] pair A,B,C,D,M,P; D=(0,0); C=(10,0); B=(10,10); A=(0,10); M=(5,0); P=(8,4); dot(M); dot(P); draw(A--B--C--D--cycle,linewidth(0.7)); draw((5,5)..D--C..cycle,linewidth(0.7)); draw((7.07,2.93)..B--A--D..cycle,linewidth(0.7)); label("$A$",A,NW); label("$B$",B,NE); label("$C$",C,SE); label("$D$",D,SW); label("$M$",M,S); label("$P$",P,N); [/asy]

$\textbf{(A)}\ 3 \qquad \textbf{(B)}\ \frac {16}{5} \qquad \textbf{(C)}\ \frac {13}{4} \qquad \textbf{(D)}\ 2\sqrt {3} \qquad \textbf{(E)}\ \frac {7}{2}$

Solutions

Solution 1

Let $D$ be the origin. $A$ is the point $(0,4)$ and $M$ is the point $(2,0)$. We are given the radius of the quarter circle and semicircle as $4$ and $2$, respectively, so their equations, respectively, are:

$x^2 + (y-4)^2 = 4^2$

$(x-2)^2 + y^2 = 2^2$

Subtract the second equation from the first:

$x^2 + (y - 4)^2 - (x - 2)^2 - y^2 = 12$

$4x - 8y + 12 = 12$

$x = 2y.$

Then substitute:

$(2y)^2 + (y - 4)^2 = 16$

$4y^2 + y^2 - 8y + 16 = 16$

$5y^2 - 8y = 0$

$y(5y - 8) = 0.$

Thus $y = 0$ and $y = \frac{8}{5}$ making $x = 0$ and $x = \frac{16}{5}$.

The first value of $0$ is obviously referring to the x-coordinate of the point where the circles intersect at the origin, $D$, so the second value must be referring to the x coordinate of $P$. Since $\overline{AD}$ is the y-axis, the distance to it from $P$ is the same as the x-value of the coordinate of $P$, so the distance from $P$ to $\overline{AD}$ is $\frac{16}{5} \Rightarrow \boxed{B}.$

Solution 2

$APMD$ obviously forms a kite. Let the intersection of the diagonals be $E$. $AE+EM=AM=2\sqrt{5}$ Let $AE=x$. Then, $EM=2\sqrt{5}-x$.


By Pythagorean Theorem, $DE^2=4^2-AE^2=2^2-EM^2$. Thus, $16-x^2=4-(2\sqrt{5}-x)^2$. Simplifying, $x=\frac{8}{\sqrt{5}}$. By Pythagoras again, $DE=\frac{4}{\sqrt{5}}$. Then, the area of $ADP$ is $DE\cdot AE=\frac{32}{5}$.


Using $4$ instead as the base, we can drop a altitude from P. $\frac{32}{5}=\frac{bh}{2}$. $\frac{32}{5}=\frac{4h}{2}$. Thus, the horizontal distance is $\frac{16}{5} \implies \boxed{\textbf{(B)}\frac{16}{5}}$


~BJHHar

Solution 3

Note that $P$ is merely a reflection of $D$ over $AM$. Call the intersection of $AM$ and $DP$ $X$. Drop perpendiculars from $X$ and $P$ to $AD$, and denote their respective points of intersection by $J$ and $K$. We then have $\triangle DXJ\sim\triangle DPK$, with a scale factor of 2. Thus, we can find $XJ$ and double it to get our answer. With some analytical geometry, we find that $XJ=\frac{8}{5}$, implying that $PK=\frac{16}{5}$.

Solution 4

As in Solution 2, draw in $DP$ and $AM$ and denote their intersection point $X$. Next, drop a perpendicular from $P$ to $AD$ and denote the foot as $Z$. $AP \cong AD$ as they are both radii and similarly $DM \cong MP$ so $APMD$ is a kite and $DX \perp XM$ by a well-known theorem.

Pythagorean theorem gives us $AM=2 \sqrt{5}$. Clearly $\triangle XMD \sim \triangle XDA \sim \triangle DMA \sim \triangle ZDP$ by angle-angle and $\triangle XMD \cong \triangle XMP$ by Hypotenuse Leg. Manipulating similar triangles gives us $PZ=\frac{16}{5}$

Solution 5

Using the double-angle formula for sine, what we need to find is $AP\cdot \sin(DAP) = AP\cdot  2\sin( DAM) \cos(DAM) = 4\cdot 2\cdot \frac{2}{\sqrt{20}}\cdot\frac{4}{\sqrt{20}} = \frac{16}{5}$.

Solution 6(LoC)

We use the Law of Cosines:

$32-32 \cos \theta = 8 + 8 \cos \theta$

$\frac{3}{5} = \cos \theta$

$2 + 2*\frac{3}{5} = \frac{16}{5}$

Solution 7

Let $H$ be the foot of the perpendicular from $P$ to $CD$, and let $HD = x$. Then we have $HC = 4-x$, and $PH = 4 - \sqrt{16 - x^2}$. Since $\triangle DHP \sim \triangle PHC$, we have $HP^2 = DH \cdot HC$, or $-x^2 + 4x = 16 - 8\sqrt{16-x^2}$. Solving gives $x = \frac{16}{5}$.

See Also

2003 AMC 12A (ProblemsAnswer KeyResources)
Preceded by
Problem 16
Followed by
Problem 18
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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