Difference between revisions of "2011 AIME I Problems/Problem 13"
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A cube with side length 10 is suspended above a plane. The vertex closest to the plane is labeled <math>A</math>. The three vertices adjacent to vertex <math>A</math> are at heights 10, 11, and 12 above the plane. The distance from vertex <math>A</math> to the plane can be expressed as <math> \frac{r-\sqrt{s}}{t}</math>, where <math>r</math>, <math>s</math>, and <math>t</math> are positive integers, and <math>r+s+t<{1000}</math>. Find <math>r+s+t</math>. | A cube with side length 10 is suspended above a plane. The vertex closest to the plane is labeled <math>A</math>. The three vertices adjacent to vertex <math>A</math> are at heights 10, 11, and 12 above the plane. The distance from vertex <math>A</math> to the plane can be expressed as <math> \frac{r-\sqrt{s}}{t}</math>, where <math>r</math>, <math>s</math>, and <math>t</math> are positive integers, and <math>r+s+t<{1000}</math>. Find <math>r+s+t</math>. | ||
| − | == | + | ==Solution== |
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Set the cube at the origin with the three vertices along the axes and the plane equal to <math>ax+by+cz+d=0</math>, where <math>a^2+b^2+c^2=1</math>. Then the (directed) distance from any point (x,y,z) to the plane is <math>ax+by+cz+d</math>. So, by looking at the three vertices, we have <math>10a+d=10, 10b+d=11, 10c+d=12</math>, and by rearranging and summing, <math>(10-d)^2+(11-d)^2+(12-d)^2= 100*(a^2+b^2+c^2)=100</math>. | Set the cube at the origin with the three vertices along the axes and the plane equal to <math>ax+by+cz+d=0</math>, where <math>a^2+b^2+c^2=1</math>. Then the (directed) distance from any point (x,y,z) to the plane is <math>ax+by+cz+d</math>. So, by looking at the three vertices, we have <math>10a+d=10, 10b+d=11, 10c+d=12</math>, and by rearranging and summing, <math>(10-d)^2+(11-d)^2+(12-d)^2= 100*(a^2+b^2+c^2)=100</math>. | ||
Solving the equation is easier if we substitute <math>11-d=y</math>, to get <math>3y^2+2=100</math>, or <math>y=\sqrt {98/3}</math>. The distance from the origin to the plane is simply d, which is equal to <math>11-\sqrt{98/3} =(33-\sqrt{294})/3</math>, so <math>33+294+3=330</math>. | Solving the equation is easier if we substitute <math>11-d=y</math>, to get <math>3y^2+2=100</math>, or <math>y=\sqrt {98/3}</math>. The distance from the origin to the plane is simply d, which is equal to <math>11-\sqrt{98/3} =(33-\sqrt{294})/3</math>, so <math>33+294+3=330</math>. | ||
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| + | == See also == | ||
| + | {{AIME box|year=2011|n=I|num-b=12|num-a=14}} | ||
Revision as of 11:52, 2 June 2011
Problem
A cube with side length 10 is suspended above a plane. The vertex closest to the plane is labeled 
. The three vertices adjacent to vertex are at heights 10, 11, and 12 above the plane. The distance from vertex to the plane can be expressed as 
, where 
, 
, and 
are positive integers, and 
. Find 
.
Solution
Set the cube at the origin with the three vertices along the axes and the plane equal to 
, where 
. Then the (directed) distance from any point (x,y,z) to the plane is 
. So, by looking at the three vertices, we have 
, and by rearranging and summing, 
.
Solving the equation is easier if we substitute 
, to get 
, or 
. The distance from the origin to the plane is simply d, which is equal to 
, so 
.
See also
| 2011 AIME I (Problems • Answer Key • Resources) | ||
| 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 | ||
