Art of Problem Solving
AoPS Online
Math texts, online classes, and more
for students in grades 5-12.
Visit AoPS Online ‚
Books for Grades 5-12 Online Courses
Beast Academy
Engaging math books and online learning
for students ages 6-13.
Visit Beast Academy ‚
Books for Ages 6-13 Beast Academy Online
AoPS Academy
Small live classes for advanced math
and language arts learners in grades 2-12.
Visit AoPS Academy ‚
Find a Physical Campus Visit the Virtual Campus

Difference between revisions of "2007 AIME I Problems/Problem 12"

m (→‎Solution)
m
Line 53: Line 53:
 
The area of <math>[\triangle EFB']</math> was found in the previous solution to be <math>- 500\sqrt{2} + 400\sqrt{3} - 300\sqrt{6} +750</math>.  
 
The area of <math>[\triangle EFB']</math> was found in the previous solution to be <math>- 500\sqrt{2} + 400\sqrt{3} - 300\sqrt{6} +750</math>.  
  
−
Therefore, <math>[ADEF] </math> <math>= (200 + 100\sqrt{3}) - \left((50 + 50\sqrt{3}) + (-500\sqrt{2} + 400\sqrt{3} - 300\sqrt{6} +750)\right)</math> <math>= 500\sqrt{2} - 350\sqrt{3} + 300\sqrt{6} - 600</math>, and our answer is <math>875</math>.
+
Therefore, <math>[ADEF] </math> <math>= (200 + 100\sqrt{3}) - \left((50 + 50\sqrt{3}) + (-500\sqrt{2} + 400\sqrt{3} - 300\sqrt{6} +750)\right)</math> <math>= 500\sqrt{2} - 350\sqrt{3} + 300\sqrt{6} - 600</math>, and our answer is <math>\boxed{875}</math>.
  
 
=== Solution 3 ===
 
=== Solution 3 ===
 
[[Image:AIME I 2007-12b.png|left]]  
 
[[Image:AIME I 2007-12b.png|left]]  
  
−
Call the points of the intersections of the triangles as <math>D</math>, <math>E</math>, and <math>F</math> as noted in the above diagram (the points are different from those in the diagram for solution 1). <math>\overline{AD}</math> [[bisect]]s <math>\angle EDE'</math>. Through [[HL]] congruency, we can find that <math>\triangle AED</math> is [[congruent]] to <math>\triangle AE'D</math>. This divides the region <math>AEDF</math> (which we are trying to solve for) into two congruent triangles and an [[isosceles triangle|isosceles]] [[right triangle]]. 
+
Call the points of the intersections of the triangles <math>D</math>, <math>E</math>, and <math>F</math> as noted in the diagram (the points are different from those in the diagram for solution 1). <math>\overline{AD}</math> [[bisect]]s <math>\angle EDE'</math>.
  
−
<math>AE = 20 \cos 15 = 20 \cos (45 - 30) = 20 \cdot \frac{\sqrt{6} + \sqrt{2}}{4} = 5\sqrt{6} + 5\sqrt{2}</math>. Since <math>FE' = AE' = AE</math>, we find that <math>[AE'F] = \frac 12 (5\sqrt{6} + 5\sqrt{2})^2 = 100 + 50\sqrt{3}</math>.  
+
Through [[HL]] congruency, we can find that <math>\triangle AED</math> is [[congruent]] to <math>\triangle AE'D</math>. This divides the region <math>AEDF</math> (which we are trying to solve for) into two congruent triangles and an [[isosceles triangle|isosceles]] [[right triangle]]. 
 +
<center><math>AE = 20 \cos 15 = 20 \cos (45 - 30) = 20 \cdot \frac{\sqrt{6} + \sqrt{2}}{4} = 5\sqrt{6} + 5\sqrt{2}</math></center>
 +
Since <math>FE' = AE' = AE</math>, we find that <math>[AE'F] = \frac 12 (5\sqrt{6} + 5\sqrt{2})^2 = 100 + 50\sqrt{3}</math>.
  
−
Now, we need to find <math>[AED] = [AE'D]</math>. The acute angles of the triangles are <math>\frac{15}{2},\ 90 - \frac{15}{2}</math>. By repeated application of the [[trigonometric identity|half-angle formula]], we can find that <math>\tan \frac{15}{2} = \sqrt{2} - \sqrt{3} + \sqrt{6} - 2</math>. The area of <math>[AED] = \frac 12 \left(20 \cos 15\right)^2 \left(\tan \frac{15}{2}\right)</math> Thus, <math>[AED] + [AE'D] = 2\left(\frac 12((5\sqrt{6} + 5\sqrt{2})^2 \cdot (\sqrt{2} - \sqrt{3} + \sqrt{6} - 2))\right)</math>. This eventually simplifies to <math>500\sqrt{2} - 350\sqrt{3} + 300\sqrt{6} - 600</math>.  
+
Now, we need to find <math>[AED] = [AE'D]</math>. The acute angles of the triangles are <math>\frac{15}{2}</math> and <math>90 - \frac{15}{2}</math>. By repeated application of the [[trigonometric identity|half-angle formula]], we can find that <math>\tan \frac{15}{2} = \sqrt{2} - \sqrt{3} + \sqrt{6} - 2</math>.
  
−
Adding them together, we find that the solution is <math>[AEDF] = [AE'F] + [AED] + [AE'D]</math> <math>= 100 + 50\sqrt{3} + 500\sqrt{2} - 350\sqrt{3} + 300\sqrt{6} - 600</math> <math>= 500\sqrt{2} - 350\sqrt{3} + 300\sqrt{6} - 600</math>.
+
The area of <math>[AED] = \frac 12 \left(20 \cos 15\right)^2 \left(\tan \frac{15}{2}\right)</math>. Thus, <math>[AED] + [AE'D] = 2\left(\frac 12((5\sqrt{6} + 5\sqrt{2})^2 \cdot (\sqrt{2} - \sqrt{3} + \sqrt{6} - 2))\right)</math>, which eventually simplifies to <math>500\sqrt{2} - 350\sqrt{3} + 300\sqrt{6} - 600</math>.
 +
 
 +
Adding them together, we find that the solution is <math>[AEDF] = [AE'F] + [AED] + [AE'D]</math> <math>= 100 + 50\sqrt{3} + 500\sqrt{2} - 350\sqrt{3} + 300\sqrt{6} - 600=</math> <math>= 500\sqrt{2} - 350\sqrt{3} + 300\sqrt{6} - 600</math>, and the answer is <math>\boxed{875}</math>.
  
 
=== Solution 4 ===
 
=== Solution 4 ===
Line 84: Line 88:
 
*<math>F</math> is the intersection of <math>\overline{AC},\ \overline{B'C'}</math>. <math>\sqrt{3}x = -\sqrt{3}x + 10(\sqrt{6} + \sqrt{2})</math>. Therefore, <math>x = \frac{15\sqrt{2} + 5\sqrt{6}}{3}</math>, and <math>y = 5\sqrt{6} + 5\sqrt{2}</math>.
 
*<math>F</math> is the intersection of <math>\overline{AC},\ \overline{B'C'}</math>. <math>\sqrt{3}x = -\sqrt{3}x + 10(\sqrt{6} + \sqrt{2})</math>. Therefore, <math>x = \frac{15\sqrt{2} + 5\sqrt{6}}{3}</math>, and <math>y = 5\sqrt{6} + 5\sqrt{2}</math>.
  
−
We take these points and tie them together by shoelace, and the answer should come out the same as <math>875</math>.
+
We take these points and tie them together by shoelace, and the answer should come out to be <math>\boxed{875}</math>.
  
 
== See also ==
 
== See also ==

Revision as of 14:51, 19 April 2008

Problem

In isosceles triangle $\triangle ABC$, $A$ is located at the origin and $B$ is located at $(20,0)$. Point $C$ is in the first quadrant with $AC = BC$ and angle $BAC = 75^{\circ}$. If triangle $ABC$ is rotated counterclockwise about point $A$ until the image of $C$ lies on the positive $y$-axis, the area of the region common to the original and the rotated triangle is in the form $p\sqrt{2} + q\sqrt{3} + r\sqrt{6} + s$, where $p,q,r,s$ are integers. Find $\frac{p-q+r-s}2$.

AIME I 2007-12.png

Solution

Solution 1

Let the new triangle be $\triangle AB'C'$ ($A$, the origin, is a vertex of both triangles). Let $\overline{B'C'}$ intersect with $\overline{AC}$ at point $D$, $\overline{BC}$ intersect with $\overline{B'C'}$ at $E$, and $\overline{BC}$ intersect with $\overline{AB'}$ at $F$. The region common to both triangles is the quadrilateral $ADEF$. Notice that $[ADEF] = [\triangle ADB'] - [\triangle EFB']$, where we let $[\ldots]$ denote area.

To find $[\triangle ADB']$:

Since $\angle B'AC'$ and $\angle BAC$ both have measures $75^{\circ}$, both of their complements are $15^{\circ}$, and $\angle DAB' = 90 - 2(15) = 60^{\circ}$. We know that $\angle DB'A = 75^{\circ}$, so $\angle ADB' = 180 - 60 - 75 = 45^{\circ}$.

Thus $\triangle ADB'$ is a $45 - 60 - 75 \triangle$. It can be solved by drawing an altitude splitting the $75^{\circ}$ angle into $30^{\circ}$ and $45^{\circ}$ angles, forming a $30-60-90$ right triangle and a $45-45-90$ isosceles right triangle. Since we know that $DB' = 20$, the base of the $30-60-90$ triangle is $10$, the base of the $45-45-90$ is $10\sqrt{3}$, and their common height is $10\sqrt{3}$. Thus, the total area of $[\triangle ADB'] = \frac{1}{2}(10\sqrt{3})(10\sqrt{3} + 10) = \boxed{150 + 50\sqrt{3}}$.

To find $[\triangle EFB']$:

Since $\triangle AFB$ is also a $15-75-90$ triangle,

$AF = 20\sin 75 = 20 \sin (30 + 45) = 20\left(\frac{\sqrt{2} + \sqrt{6}}4\right) = 5\sqrt{2} + 5\sqrt{6}$

and

$FB' = AB' - AF = 20 - 5\sqrt{2} - 5\sqrt{6}$

Since $[\triangle EFB'] = \frac{1}{2} (FB' \cdot EF) = \frac{1}{2} (FB') (FB' \tan 75^{\circ})$. With some horrendous algebra, we can calculate

$\begin{align*}

[\triangle EFB'] & = \frac{1}{2}\tan (30 + 45) \cdot (20 - 5\sqrt{2} - 5\sqrt{6})^2 \\ & = 25 \left(\frac{\frac{1}{\sqrt{3}} + 1}{1 - \frac{1}{\sqrt{3}}}\right) \left(8 - 2\sqrt{2} - 2\sqrt{6} - 2\sqrt{2} + 1 + \sqrt{3} - 2\sqrt{6} + \sqrt{3} + 3\right) \\ & = 25(2 + \sqrt{3})(12 - 4\sqrt{2} - 4\sqrt{6} + 2\sqrt{3}) \\ [\triangle EFB'] & = \boxed{- 500\sqrt{2} + 400\sqrt{3} - 300\sqrt{6} +750}.

\end{align*}$ (Error compiling LaTeX. Unknown error_msg)

To finish,

$\begin{align*}

[ADEF] &= [\triangle ADB'] - [\triangle EFB']\\ 

      &= \left(150 + 50\sqrt{3}\right) - \left(-500\sqrt{2} + 400\sqrt{3} - 300\sqrt{6} + 750\right)\\
&=500\sqrt{2} - 350\sqrt{3} + 300\sqrt{6} - 600\\ \end{align*}$ (Error compiling LaTeX. Unknown error_msg)

Hence, $\frac{p-q+r-s}{2} = \frac{500 + 350 + 300 + 600}2 = \frac{1750}2 = \boxed{\boxed{875}}$.

Solution 2

Redefine the points in the same manner as the last time ($\triangle AB'C'$, intersect at $D$, $E$, and $F$). This time, notice that $[ADEF] = [\triangle AB'C'] - ([\triangle ADC'] + [\triangle EFB']$.

The area of $[\triangle AB'C'] = [\triangle ABC]$. The altitude of $\triangle ABC$ is clearly $10 \tan 75 = 10 \tan (30 + 45)$. The tangent addition rule yields $10(2 + \sqrt{3})$ (see above). Thus, $[\triangle ABC] = \frac{1}{2} 20 \cdot (20 + 10\sqrt{3}) = 200 + 100\sqrt{3}$.

The area of $[\triangle ADC']$ (with a side on the y-axis) can be found by splitting it into two triangles, $30-60-90$ and $15-75-90$ right triangles. $AC' = AC = \frac{10}{\sin 15}$. The sine subtraction rule shows that $\frac{10}{\sin 15} = \frac{10}{\frac{\sqrt{6} - \sqrt{2}}4} = \frac{40}{\sqrt{6} - \sqrt{2}} = 10(\sqrt{6} + \sqrt{2})$. $AC'$, in terms of the height of $\triangle ADC'$, is equal to $h(\sqrt{3} + \tan 75) = h(\sqrt{3} + 2 + \sqrt{3})$.

$\begin{align*}

[ADC'] &= \frac 12 AC' \cdot h \\ &= \frac 12 (10\sqrt{6} + 10\sqrt{2})\left(\frac{10\sqrt{6} + 10\sqrt{2}}{2\sqrt{3} + 2}\right) \\ &= \frac{(800 + 400\sqrt{3})}{(2 + \sqrt{3})}\cdot\frac{2 - \sqrt{3}}{2-\sqrt{3}} \\ &= \frac{400\sqrt{3} + 400}8 = 50\sqrt{3} + 50

\end{align*}$ (Error compiling LaTeX. Unknown error_msg)

The area of $[\triangle EFB']$ was found in the previous solution to be $- 500\sqrt{2} + 400\sqrt{3} - 300\sqrt{6} +750$.

Therefore, $[ADEF]$ $= (200 + 100\sqrt{3}) - \left((50 + 50\sqrt{3}) + (-500\sqrt{2} + 400\sqrt{3} - 300\sqrt{6} +750)\right)$ $= 500\sqrt{2} - 350\sqrt{3} + 300\sqrt{6} - 600$, and our answer is $\boxed{875}$.

Solution 3

AIME I 2007-12b.png

Call the points of the intersections of the triangles $D$, $E$, and $F$ as noted in the diagram (the points are different from those in the diagram for solution 1). $\overline{AD}$ bisects $\angle EDE'$.

Through HL congruency, we can find that $\triangle AED$ is congruent to $\triangle AE'D$. This divides the region $AEDF$ (which we are trying to solve for) into two congruent triangles and an isosceles right triangle.

$AE = 20 \cos 15 = 20 \cos (45 - 30) = 20 \cdot \frac{\sqrt{6} + \sqrt{2}}{4} = 5\sqrt{6} + 5\sqrt{2}$

Since $FE' = AE' = AE$, we find that $[AE'F] = \frac 12 (5\sqrt{6} + 5\sqrt{2})^2 = 100 + 50\sqrt{3}$.

Now, we need to find $[AED] = [AE'D]$. The acute angles of the triangles are $\frac{15}{2}$ and $90 - \frac{15}{2}$. By repeated application of the half-angle formula, we can find that $\tan \frac{15}{2} = \sqrt{2} - \sqrt{3} + \sqrt{6} - 2$.

The area of $[AED] = \frac 12 \left(20 \cos 15\right)^2 \left(\tan \frac{15}{2}\right)$. Thus, $[AED] + [AE'D] = 2\left(\frac 12((5\sqrt{6} + 5\sqrt{2})^2 \cdot (\sqrt{2} - \sqrt{3} + \sqrt{6} - 2))\right)$, which eventually simplifies to $500\sqrt{2} - 350\sqrt{3} + 300\sqrt{6} - 600$.

Adding them together, we find that the solution is $[AEDF] = [AE'F] + [AED] + [AE'D]$ $= 100 + 50\sqrt{3} + 500\sqrt{2} - 350\sqrt{3} + 300\sqrt{6} - 600=$ $= 500\sqrt{2} - 350\sqrt{3} + 300\sqrt{6} - 600$, and the answer is $\boxed{875}$.

Solution 4

From the given information, calculate the coordinates of all the points (by finding the equations of the lines, equating). Then, use the shoelace method to calculate the area of the intersection.

  • $\overline{AC}$: $y = \sqrt{3}x$
  • $\overline{AB'}$: $y = (\tan 15) x = \frac{\sqrt{6} - \sqrt{2}}{4}x$
  • $\overline{BC}$: It passes thru $(20,0)$, and has a slope of $-\tan75 = -\frac{\sqrt{6} + \sqrt{2}}{4}$. The equation of its line is $y = -\frac{\sqrt{6} + \sqrt{2}}{4}x + 5\sqrt{6} + 5\sqrt{2}$.
  • $\overline{B'C'}$: $AC' = AC = \frac{10}{\sin 75} = 10\sqrt{6} + 10\sqrt{2}$, so it passes thru point $(0, 10\sqrt{6} + 10\sqrt{2})$. It has a slope of $-\tan 60 = -\sqrt{3}$. So the equation of its line is $y = -\sqrt{3}x + 10(\sqrt{6} + \sqrt{2})$.

Now, we can equate the equations to find the intersections of all the points.

  • $A (0,0)$
  • $D$ is the intersection of $\overline{AB'},\ \overline{B'C'}$. $\frac{\sqrt{6} - \sqrt{2}}{4}x = -\sqrt{3}x + 10(\sqrt{6} + \sqrt{2})$. Therefore, $x = \frac{40\sqrt{6} + 40\sqrt{2}}{\sqrt{6} - \sqrt{2} + 4\sqrt{3}}$.
  • $E$ is the intersection of $\overline{AB'},\ \overline{BC}$. $\frac{\sqrt{6} - \sqrt{2}}{4}x = -\frac{\sqrt{6} + \sqrt{2}}{4}x + 5\sqrt{6} + 5\sqrt{2}$. Therefore, $x = 10 + \frac{10\sqrt{3}}{3}$.
  • $F$ is the intersection of $\overline{AC},\ \overline{B'C'}$. $\sqrt{3}x = -\sqrt{3}x + 10(\sqrt{6} + \sqrt{2})$. Therefore, $x = \frac{15\sqrt{2} + 5\sqrt{6}}{3}$, and $y = 5\sqrt{6} + 5\sqrt{2}$.

We take these points and tie them together by shoelace, and the answer should come out to be $\boxed{875}$.

See also

2007 AIME I (Problems • Answer Key • Resources)
Preceded by
Problem 11 		Followed by
Problem 13
1 • 2 • 3 • 4 • 5 • 6 • 7 • 8 • 9 • 10 • 11 • 12 • 13 • 14 • 15
All AIME Problems and Solutions
Retrieved from "https://artofproblemsolving.com/wiki/index.php?title=2007_AIME_I_Problems/Problem_12&oldid=25046"