Difference between revisions of "2007 AIME I Problems/Problem 13"

(Problem)
(Solution 1)
Line 10: Line 10:
 
Use 3D analytical geometry, setting the origin as the center of the square base and the pyramid’s points oriented as shown above. <math>A(-2,2,0),\ B(2,2,0),\ C(2,-2,0),\ D(-2,-2,0),\ E(0,0,2\sqrt{2})</math>. Using the coordinates of the three points of intersection (<math>(-1,1,\sqrt{2}),\ (2,0,0),\ (0,-2,0)</math>), it is possible to determine the equation of the plane. The equation of a plane resembles <math>ax + by + cz = d</math>, and using the points we find that <math>2a = d \Longrightarrow d = \frac{a}{2}</math>, <math>-2b = d \Longrightarrow d = \frac{-b}{2}</math>, and <math>-a + b + \sqrt{2}c = d \Longrightarrow -\frac{d}{2} - \frac{d}{2} + \sqrt{2}c = d \Longrightarrow c = d\sqrt{2}</math>. It is then <math>x - y + 2\sqrt{2}z = 2</math>.
 
Use 3D analytical geometry, setting the origin as the center of the square base and the pyramid’s points oriented as shown above. <math>A(-2,2,0),\ B(2,2,0),\ C(2,-2,0),\ D(-2,-2,0),\ E(0,0,2\sqrt{2})</math>. Using the coordinates of the three points of intersection (<math>(-1,1,\sqrt{2}),\ (2,0,0),\ (0,-2,0)</math>), it is possible to determine the equation of the plane. The equation of a plane resembles <math>ax + by + cz = d</math>, and using the points we find that <math>2a = d \Longrightarrow d = \frac{a}{2}</math>, <math>-2b = d \Longrightarrow d = \frac{-b}{2}</math>, and <math>-a + b + \sqrt{2}c = d \Longrightarrow -\frac{d}{2} - \frac{d}{2} + \sqrt{2}c = d \Longrightarrow c = d\sqrt{2}</math>. It is then <math>x - y + 2\sqrt{2}z = 2</math>.
  
<center> <asy>import three; pointpen = black; pathpen = black+linewidth(0.7); currentprojection = perspective(2.5,-12,4);
+
<center>  
 +
<asy>import three;  
 +
pointpen = black;  
 +
pathpen = black+linewidth(0.7);  
 +
currentprojection = perspective(2.5,-12,4);
 
triple A=(-2,2,0), B=(2,2,0), C=(2,-2,0), D=(-2,-2,0), E=(0,0,2*2^.5), P=(A+E)/2, Q=(B+C)/2, R=(C+D)/2, Y=(-3/2,-3/2,2^.5/2),X=(3/2,3/2,2^.5/2);
 
triple A=(-2,2,0), B=(2,2,0), C=(2,-2,0), D=(-2,-2,0), E=(0,0,2*2^.5), P=(A+E)/2, Q=(B+C)/2, R=(C+D)/2, Y=(-3/2,-3/2,2^.5/2),X=(3/2,3/2,2^.5/2);
D(A--B--C--D--A--E--B--E--C--E--D); MP("A",A); MP("B",B,(1,0,0)); MP("C",C); MP("D",D); MP("E",E,N); D(MP("P",P)); D(MP("Q",Q,(1,0,0))); D(MP("R",R));  D(MP("Y",Y,NW));  D(MP("X",X,NE)); D(P--X--Q--R--Y--cycle,linetype("6 6")+linewidth(0.7));  
+
draw(A--B--C--D--A--E--B--E--C--E--D);  
</asy>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;<asy>pointpen = black; pathpen = black+linewidth(0.7);  
+
label("A",A, SE);  
 +
label("B",B,(1,0,0));  
 +
label("C",C, SE);  
 +
label("D",D, W);  
 +
label("E",E,N);  
 +
label("P",P, NW);  
 +
label("Q",Q,(1,0,0));  
 +
label("R",R, S);   
 +
label("Y",Y,NW);   
 +
label("X",X,NE);  
 +
draw(P--X--Q--R--Y--cycle,linetype("6 6")+linewidth(0.7));  
 +
</asy>
 +
<asy>
 +
pointpen = black;  
 +
pathpen = black+linewidth(0.7);  
 
pair P = (0, 2.5^.5), X = (3/2^.5,0), Y = (-3/2^.5,0), Q = (2^.5,-2.5^.5), R = (-2^.5,-2.5^.5);  
 
pair P = (0, 2.5^.5), X = (3/2^.5,0), Y = (-3/2^.5,0), Q = (2^.5,-2.5^.5), R = (-2^.5,-2.5^.5);  
D(MP("P",P,N)--MP("X",X,NE)--MP("Q",Q)--MP("R",R)--MP("Y",Y,NW)--cycle); D(X--Y,linetype("6 6") + linewidth(0.7)); D(P--(0,-P.y),linetype("6 6") + linewidth(0.7));
+
D(MP("P",P,N)--MP("X",X,NE)--MP("Q",Q)--MP("R",R)--MP("Y",Y,NW)--cycle);  
MP("3\sqrt{2}",(X+Y)/2); MP("2\sqrt{2}",(Q+R)/2); MP("\sqrt{\frac{5}{2}}",(0,-P.y/2),E); MP("\sqrt{\frac{5}{2}}",(0,2*P.y/5),E);  
+
D(X--Y,linetype("6 6") + linewidth(0.7)); D(P--(0,-P.y),linetype("6 6") + linewidth(0.7));
 +
MP("3\sqrt{2}",(X+Y)/2);  
 +
MP("2\sqrt{2}",(Q+R)/2);  
 +
MP("\sqrt{\frac{5}{2}}",(0,-P.y/2),E);  
 +
MP("\sqrt{\frac{5}{2}}",(0,2*P.y/5),E);  
 
</asy>
 
</asy>
 
 
</center>
 
</center>
  

Revision as of 21:15, 25 August 2015

Problem

A square pyramid with base $ABCD$ and vertex $E$ has eight edges of length $4$. A plane passes through the midpoints of $AE$, $BC$, and $CD$. The plane's intersection with the pyramid has an area that can be expressed as $\sqrt{p}$. Find $p$.

AIME I 2007-13.png

Solution

Solution 1

Note first that the intersection is a pentagon.

Use 3D analytical geometry, setting the origin as the center of the square base and the pyramid’s points oriented as shown above. $A(-2,2,0),\ B(2,2,0),\ C(2,-2,0),\ D(-2,-2,0),\ E(0,0,2\sqrt{2})$. Using the coordinates of the three points of intersection ($(-1,1,\sqrt{2}),\ (2,0,0),\ (0,-2,0)$), it is possible to determine the equation of the plane. The equation of a plane resembles $ax + by + cz = d$, and using the points we find that $2a = d \Longrightarrow d = \frac{a}{2}$, $-2b = d \Longrightarrow d = \frac{-b}{2}$, and $-a + b + \sqrt{2}c = d \Longrightarrow -\frac{d}{2} - \frac{d}{2} + \sqrt{2}c = d \Longrightarrow c = d\sqrt{2}$. It is then $x - y + 2\sqrt{2}z = 2$.

[asy]import three;  pointpen = black;  pathpen = black+linewidth(0.7);  currentprojection = perspective(2.5,-12,4); triple A=(-2,2,0), B=(2,2,0), C=(2,-2,0), D=(-2,-2,0), E=(0,0,2*2^.5), P=(A+E)/2, Q=(B+C)/2, R=(C+D)/2, Y=(-3/2,-3/2,2^.5/2),X=(3/2,3/2,2^.5/2); draw(A--B--C--D--A--E--B--E--C--E--D);  label("A",A, SE);  label("B",B,(1,0,0));  label("C",C, SE);  label("D",D, W);  label("E",E,N);  label("P",P, NW);  label("Q",Q,(1,0,0));  label("R",R, S);   label("Y",Y,NW);   label("X",X,NE);  draw(P--X--Q--R--Y--cycle,linetype("6 6")+linewidth(0.7));  [/asy] [asy] pointpen = black;  pathpen = black+linewidth(0.7);  pair P = (0, 2.5^.5), X = (3/2^.5,0), Y = (-3/2^.5,0), Q = (2^.5,-2.5^.5), R = (-2^.5,-2.5^.5);  D(MP("P",P,N)--MP("X",X,NE)--MP("Q",Q)--MP("R",R)--MP("Y",Y,NW)--cycle);  D(X--Y,linetype("6 6") + linewidth(0.7)); D(P--(0,-P.y),linetype("6 6") + linewidth(0.7)); MP("3\sqrt{2}",(X+Y)/2);  MP("2\sqrt{2}",(Q+R)/2);  MP("\sqrt{\frac{5}{2}}",(0,-P.y/2),E);  MP("\sqrt{\frac{5}{2}}",(0,2*P.y/5),E);  [/asy]

Write the equation of the lines and substitute to find that the other two points of intersection on $\overline{BE}$, $\overline{DE}$ are $\left(\frac{\pm 3}{2},\frac{\pm 3}{2},\frac{\sqrt{2}}{2}\right)$. To find the area of the pentagon, break it up into pieces (an isosceles triangle on the top, an isosceles trapezoid on the bottom). Using the distance formula ($\sqrt{a^2 + b^2 + c^2}$), it is possible to find that the area of the triangle is $\frac{1}{2}bh \Longrightarrow \frac{1}{2} 3\sqrt{2} \cdot \sqrt{\frac 52} = \frac{3\sqrt{5}}{2}$. The trapezoid has area $\frac{1}{2}h(b_1 + b_2) \Longrightarrow \frac 12\sqrt{\frac 52}\left(2\sqrt{2} + 3\sqrt{2}\right) = \frac{5\sqrt{5}}{2}$. In total, the area is $4\sqrt{5} = \sqrt{80}$, and the solution is $\boxed{080}$.

Solution 2

Use the same coordinate system as above, and let the plane determined by $\triangle PQR$ intersect $\overline{BE}$ at $X$ and $\overline{DE}$ at $Y$. Then the line $\overline{XY}$ is the intersection of the planes determined by $\triangle PQR$ and $\triangle BDE$.

Note that the plane determined by $\triangle BDE$ has the equation $x=y$, and $\overline{PQ}$ can be described by $x=2(1-t)-t,\ y=t,\ z=t\sqrt{2}$. It intersects the plane when $2(1-t)-t=t$, or $t=\frac{1}{2}$. This intersection point has $z=\frac{\sqrt{2}}{2}$. Similarly, the intersection between $\overline{PR}$ and $\triangle BDE$ has $z=\frac{\sqrt{2}}{2}$. So $\overline{XY}$ lies on the plane $z=\frac{\sqrt{2}}{2}$, from which we obtain $X=\left( \frac{3}{2},\frac{3}{2},\frac{\sqrt{2}}{2}\right)$ and $Y=\left( -\frac{3}{2},-\frac{3}{2},\frac{\sqrt{2}}{2}\right)$. The area of the pentagon $EXQRY$ can be computed in the same way as above.

See also

2007 AIME I (ProblemsAnswer KeyResources)
Preceded by
Problem 12
Followed by
Problem 14
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15
All AIME Problems and Solutions

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