Difference between revisions of "2006 AIME I Problems/Problem 14"
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== Problem == | == Problem == | ||
− | A tripod has three legs each of length 5 feet. When the tripod is set up, the angle between any pair of legs is equal to the angle between any other pair, and the top of the tripod is 4 feet from the ground In setting up the tripod, the lower 1 foot of one leg breaks off. Let <math> h </math> be the height in feet of the top of the tripod from the ground when the broken tripod is set up. Then <math> h </math> can be written in the form <math> \frac m{\sqrt{n}}, </math> where <math> m </math> and <math> n </math> are positive integers and <math> n </math> is not divisible by the square of any prime. Find <math> \lfloor m+\sqrt{n}\rfloor. </math> (The notation <math> \lfloor x\rfloor </math> denotes the greatest integer that is less than or equal to <math> x. </math>) | + | A tripod has three legs each of length <math>5</math> feet. When the tripod is set up, the [[angle]] between any pair of legs is equal to the angle between any other pair, and the top of the tripod is <math>4</math> feet from the ground. In setting up the tripod, the lower 1 foot of one leg breaks off. Let <math> h </math> be the height in feet of the top of the tripod from the ground when the broken tripod is set up. Then <math> h </math> can be written in the form <math> \frac m{\sqrt{n}}, </math> where <math> m </math> and <math> n </math> are positive integers and <math> n </math> is not divisible by the square of any prime. Find <math> \lfloor m+\sqrt{n}\rfloor. </math> (The notation <math> \lfloor x\rfloor </math> denotes the greatest integer that is less than or equal to <math> x. </math>) |
− | == Solution == | + | == Solution 1 == |
− | {{ | + | <center><asy> |
+ | size(200); | ||
+ | import three; pointpen=black;pathpen=black+linewidth(0.65);pen ddash = dashed+linewidth(0.65); | ||
+ | currentprojection = perspective(1,-10,3.3); | ||
+ | triple O=(0,0,0),T=(0,0,5),C=(0,3,0),A=(-3*3^.5/2,-3/2,0),B=(3*3^.5/2,-3/2,0); | ||
+ | triple M=(B+C)/2,S=(4*A+T)/5; | ||
+ | draw(T--S--B--T--C--B--S--C);draw(B--A--C--A--S,ddash);draw(T--O--M,ddash); | ||
+ | label("$T$",T,N);label("$A$",A,SW);label("$B$",B,SE);label("$C$",C,NE);label("$S$",S,NW);label("$O$",O,SW);label("$M$",M,NE); | ||
+ | label("$4$",(S+T)/2,NW);label("$1$",(S+A)/2,NW);label("$5$",(B+T)/2,NE);label("$4$",(O+T)/2,W); | ||
+ | dot(S);dot(O); | ||
+ | </asy></center> | ||
+ | |||
+ | We will use <math>[...]</math> to denote volume (four letters), area (three letters) or length (two letters). | ||
+ | |||
+ | Let <math>T</math> be the top of the tripod, <math>A,B,C</math> are end points of three legs. Let <math>S</math> be the point on <math>TA</math> such that <math>[TS] = 4</math> and <math>[SA] = 1</math>. Let <math>O</math> be the center of the base [[equilateral triangle]] <math>ABC</math>. Let <math>M</math> be the [[midpoint]] of segment <math>BC</math>. Let <math>h</math> be the distance from <math>T</math> to the triangle <math>SBC</math> (<math>h</math> is what we want to find). | ||
+ | |||
+ | We have the volume ratio <math>\frac {[TSBC]}{[TABC]} = \frac {[TS]}{[TA]} = \frac {4}{5}</math>. | ||
+ | |||
+ | So <math>\frac {h\cdot [SBC]}{[TO]\cdot [ABC]} = \frac {4}{5}</math>. | ||
+ | |||
+ | We also have the [[area ratios|area ratio]] <math>\frac {[SBC]}{[ABC]} = \frac {[SM]}{[AM]}</math>. | ||
+ | |||
+ | The triangle <math>TOA</math> is a <math>3-4-5</math> [[right triangle]] so <math>[AM] = \frac {3}{2}\cdot[AO] = \frac {9}{2}</math> and <math>\cos{\angle{TAO}} = \frac {3}{5}</math>. | ||
+ | |||
+ | Applying [[Law of Cosines]] to the triangle <math>SAM</math> with <math>[SA] = 1</math>, <math>[AM] = \frac {9}{2}</math> and <math>\cos{\angle{SAM}} = \frac {3}{5}</math>, we find: | ||
+ | |||
+ | <center><math>[SM] = \frac {\sqrt {5\cdot317}}{10}.</math></center> | ||
+ | |||
+ | Putting it all together, we find <math>h = \frac {144}{\sqrt {5\cdot317}}</math>. | ||
+ | |||
+ | <center><math> \lfloor 144+\sqrt{5 \cdot 317}\rfloor =144+ \lfloor \sqrt{5 \cdot 317}\rfloor =144+\lfloor \sqrt{1585} \rfloor =144+39=\boxed{183}</math>.</center> | ||
+ | |||
+ | == Solution 2 == | ||
+ | |||
+ | We note that <math>AO=3</math>. From this we can derive that the side length of the equilateral is <math>3\sqrt{3}</math>. We now use 3D coordinate geometry. | ||
+ | <cmath>A = (0,0,0)</cmath> | ||
+ | <cmath>B = (3\sqrt{3},0,0)</cmath> | ||
+ | <cmath>C = (\frac{3\sqrt{3}}{2}, \frac{9}{2}, 0)</cmath> | ||
+ | <cmath>T = (\frac{3\sqrt{3}}{2}, \frac{3}{2}, 4)</cmath> | ||
+ | <cmath>S= (\frac{3\sqrt{3}}{10}, \frac{3}{10}, \frac{4}{5})</cmath> | ||
+ | We know three points of plane <math>SCB</math> hence we can write out the equation for the plane. | ||
+ | Plane <math>SCB</math> can be expressed as <cmath>4\sqrt{3}x+4y+39z-36=0.</cmath> | ||
+ | |||
+ | Applying the distance between a point and a plane formula. | ||
+ | |||
+ | <cmath>\frac{ax+by+cz+d}{\sqrt{a^{2}+b^{2}+c^{2}}} = \frac{4\sqrt{3} \cdot \frac{3\sqrt{3}}{2} + 4\cdot \frac{3}{2} + 39 \cdot 4 -36}{\sqrt{(4\sqrt{3})^2+4^2+39^2}} = \frac{144}{\sqrt{1585}}</cmath> | ||
+ | |||
+ | <cmath>\lfloor m+\sqrt{n}\rfloor = \lfloor 144+\sqrt{1585}\rfloor = 183 </cmath> | ||
+ | |||
+ | Solution by SimonSun | ||
+ | |||
+ | |||
+ | == Solution 3 (Cosine Law & Pythagorean Bash) == | ||
+ | |||
+ | Diagram borrowed from Solution 1 | ||
+ | <center><asy> | ||
+ | size(200); | ||
+ | import three; pointpen=black;pathpen=black+linewidth(0.65);pen ddash = dashed+linewidth(0.65); | ||
+ | currentprojection = perspective(1,-10,3.3); | ||
+ | triple O=(0,0,0),T=(0,0,5),C=(0,3,0),A=(-3*3^.5/2,-3/2,0),B=(3*3^.5/2,-3/2,0); | ||
+ | triple M=(B+C)/2,S=(4*A+T)/5; | ||
+ | draw(T--S--B--T--C--B--S--C);draw(B--A--C--A--S,ddash);draw(T--O--M,ddash); | ||
+ | label("$T$",T,N);label("$A$",A,SW);label("$B$",B,SE);label("$C$",C,NE);label("$S$",S,NW);label("$O$",O,SW);label("$M$",M,NE); | ||
+ | label("$4$",(S+T)/2,NW);label("$1$",(S+A)/2,NW);label("$5$",(B+T)/2,NE);label("$4$",(O+T)/2,W); | ||
+ | dot(S);dot(O); | ||
+ | </asy></center> | ||
+ | |||
+ | Apply [[Pythagorean Theorem]] on <math>\bigtriangleup TOB</math> yields | ||
+ | <cmath>BO=\sqrt{TB^2-TO^2}=3</cmath> | ||
+ | Since <math>\bigtriangleup ABC</math> is equilateral, we have <math>\angle MOB=60^{\circ}</math> and | ||
+ | <cmath>BC=2BM=2(OB\sin MOB)=3\sqrt{3}</cmath> | ||
+ | Apply Pythagorean Theorem on <math>\bigtriangleup TMB</math> yields | ||
+ | <cmath>TM=\sqrt{TB^2-BM^2}=\sqrt{5^2-(\frac{3\sqrt{3}}{2})^2}=\frac{\sqrt{73}}{2}</cmath> | ||
+ | Apply [[Law of Cosines]] on <math>\bigtriangleup TBC</math> we have | ||
+ | <cmath>BC^2=TB^2+TC^2-2(TB)(TC)\cos BTC</cmath> | ||
+ | <cmath>(3\sqrt{3})^2=5^2+5^2-2(5)^2\cos BTC</cmath> | ||
+ | <cmath>\cos BTC=\frac{23}{50}</cmath> | ||
+ | Apply Law of Cosines on <math>\bigtriangleup STB</math> using the fact that <math>\angle STB=\angle BTC</math> we have | ||
+ | <cmath>SB^2=ST^2+BT^2-2(ST)(BT)\cos STB</cmath> | ||
+ | <cmath>SB=\sqrt{4^2+5^2-2(4)(5)\cos BTC}=\frac{\sqrt{565}}{5}</cmath> | ||
+ | Apply Pythagorean Theorem on <math>\bigtriangleup BSM</math> yields | ||
+ | <cmath>SM=\sqrt{SB^2-BM^2}=\frac{\sqrt{1585}}{10}</cmath> | ||
+ | Let the perpendicular from <math>T</math> hits <math>SBC</math> at <math>P</math>. Let <math>SP=x</math> and <math>PM=\frac{\sqrt{1585}}{10}-x</math>. Apply Pythagorean Theorem on <math>TSP</math> and <math>TMP</math> we have | ||
+ | <cmath>TP^2=TS^2-SP^2=TM^2-PM^2</cmath> | ||
+ | <cmath>4^2-x^2=(\frac{\sqrt{73}}{2})^2-(\frac{\sqrt{1585}}{10}-x)^2</cmath> | ||
+ | Cancelling out the <math>x^2</math> term and solving gets <math>x=\frac{181}{2\sqrt{1585}}</math>. | ||
+ | |||
+ | Finally, by Pythagorean Theorem, | ||
+ | <cmath>TP=\sqrt{TS^2-SP^2}=\frac{144}{\sqrt{1585}}</cmath> | ||
+ | so <math>\lfloor m+\sqrt{n}\rfloor=\boxed{183}</math> | ||
+ | |||
+ | ~ Nafer | ||
== See also == | == See also == | ||
Line 9: | Line 100: | ||
[[Category:Intermediate Geometry Problems]] | [[Category:Intermediate Geometry Problems]] | ||
+ | {{MAA Notice}} |
Latest revision as of 20:45, 31 March 2020
Contents
Problem
A tripod has three legs each of length feet. When the tripod is set up, the angle between any pair of legs is equal to the angle between any other pair, and the top of the tripod is feet from the ground. In setting up the tripod, the lower 1 foot of one leg breaks off. Let be the height in feet of the top of the tripod from the ground when the broken tripod is set up. Then can be written in the form where and are positive integers and is not divisible by the square of any prime. Find (The notation denotes the greatest integer that is less than or equal to )
Solution 1
We will use to denote volume (four letters), area (three letters) or length (two letters).
Let be the top of the tripod, are end points of three legs. Let be the point on such that and . Let be the center of the base equilateral triangle . Let be the midpoint of segment . Let be the distance from to the triangle ( is what we want to find).
We have the volume ratio .
So .
We also have the area ratio .
The triangle is a right triangle so and .
Applying Law of Cosines to the triangle with , and , we find:
Putting it all together, we find .
Solution 2
We note that . From this we can derive that the side length of the equilateral is . We now use 3D coordinate geometry. We know three points of plane hence we can write out the equation for the plane. Plane can be expressed as
Applying the distance between a point and a plane formula.
Solution by SimonSun
Solution 3 (Cosine Law & Pythagorean Bash)
Diagram borrowed from Solution 1
Apply Pythagorean Theorem on yields Since is equilateral, we have and Apply Pythagorean Theorem on yields Apply Law of Cosines on we have Apply Law of Cosines on using the fact that we have Apply Pythagorean Theorem on yields Let the perpendicular from hits at . Let and . Apply Pythagorean Theorem on and we have Cancelling out the term and solving gets .
Finally, by Pythagorean Theorem, so
~ Nafer
See also
2006 AIME I (Problems • Answer Key • Resources) | ||
Preceded by Problem 13 |
Followed by Problem 15 | |
1 • 2 • 3 • 4 • 5 • 6 • 7 • 8 • 9 • 10 • 11 • 12 • 13 • 14 • 15 | ||
All AIME Problems and Solutions |
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