2007 AIME I Problems/Problem 12

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 $\displaystyle AC = BC$ 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 $\displaystyle p,q,r,s$ are integers. Find $\frac{p-q+r-s}2$ .

Solution

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Solution 1

Call the vertices of the new triangle $\displaystyle AB'C'$ ( $A$ , the origin, is a vertex of both triangles). $\displaystyle B'C'$ and $\displaystyle AB$ intersect at a single point, $D$ . $\displaystyle BC$ intersect at two points; the one with the higher y-coordinate will be $E$ , and the other $F$ . The intersection of the two triangles is a quadrilateral $\displaystyle ADEF$ . Notice that we can find this area by subtracting $[\triangle ADB'] - [\triangle EFB']$ .

Since $\displaystyle \angle B'AC'$ and $\displaystyle \angle BAC$ both have measures $75^{\circ}$ , both of their complements are $15^{\circ}$ , and $\angle DAC' = 90 - 2(15) = 60^{\circ}$ . We know that $C'B'A = 75^{\circ}$ , and since the angles of a triangle add up to $180^{\circ}$ , we find that $ADB' = 180 - 60 - 75 = 45^{\circ}$ .

So $\displaystyle ADB'$ 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 – this forms a $\displaystyle 30-60-90$ right triangle and a $\displaystyle 45-45-90$ isosceles right triangle. Since we know that $\displaystyle DB' = 20$ , the base of the $\displaystyle 30-60-90$ triangle is $10$ , the height is $10\sqrt{3}$ , and the base of the $\displaystyle 45-45-90$ is $10\sqrt{3}$ . Thus, the total area of $[\triangle ADB'] = \frac{1}{2}(10\sqrt{3})(10\sqrt{3} + 10) = 150 + 50\sqrt{3}$ .

Now, we need to find $[\triangle EFB']$ , which is a $\displaystyle 15-75-90$ right triangle. We can find its base by subtracting $AF$ from $20$ . $\triangle AFB$ is also a $\displaystyle 15-75-90$ triangle, so we find that $AF = 20\sin 75 = 20 \sin (30 + 45) = 20\frac{\sqrt{2} + \sqrt{6}}4 = 5\sqrt{2} + 5\sqrt{6}$ . $FB' = 20 - AF = 20 - 5\sqrt{2} - 5\sqrt{6}$ .

To solve $[\triangle EFB']$ , note that $\displaystyle [\triangle EFB'] = \frac{1}{2} FB' \cdot EF = \frac{1}{2} FB' \cdot (\tan 75 FB')$ . Through algebra, we can calculate $(FB')^2 \cdot \tan 75$ :

$\displaystyle \frac{1}{2}\tan (30 + 45) \cdot (20 - 5\sqrt{2} - 5\sqrt{6})^2$

$\displaystyle = \frac{1}{2} \left(\frac{\frac{1}{\sqrt{3}} + 1}{1 - \frac{1}{\sqrt{3}}}\right) \left[400 - 100\sqrt{2} - 100\sqrt{6} - 100\sqrt{2} + 50 + 50\sqrt{3} - 100\sqrt{6} + 50\sqrt{3} + 150\right]$

$= \frac{1}{2}(2 + \sqrt{3})[600 - 200\sqrt{2} - 200\sqrt{6} + 100\sqrt{3}]$

$=- 500\sqrt{2} + 400\sqrt{3} - 300\sqrt{6} +750$

To finish, find $[ADEF] = [\triangle ADB'] - [\triangle EFB']$ $= \left(150 + 50\sqrt{3}\right) - \left(-500\sqrt{2} + 400\sqrt{3} - 300\sqrt{6} + 750\right)$ $=500\sqrt{2} - 350\sqrt{3} + 300\sqrt{6} - 600$ . The solution is $\frac{500 + 350 + 300 + 600}2 = \frac{1750}2 = 875$ .

Solution 2

Redefine the points in the same manner as the last time ( $\displaystyle \triangle AB'C'$ , 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, $\displaystyle [\triangle ABC] = \frac{1}{2} 20 \cdot (20 + 10\sqrt{3}) = 200 + 100\sqrt{3}$ .

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. $AC' = AC = \frac{10}{\sin 75}$ . 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 $h(\sqrt{3} + \tan 75) = h(\sqrt{3} + 2 + \sqrt{3})$ . $[ADC'] = \frac 12 AC' \cdot h = \frac 12 (10\sqrt{6} + 10\sqrt{2})(\frac{10\sqrt{6} + 10\sqrt{2}}{2\sqrt{3} + 2}$ $= \frac{(800 + 400\sqrt{3})}{(2 + \sqrt{3})}\cdot\frac{2 - \sqrt{3}}{2-\sqrt{3}}$ $= \frac{400\sqrt{3} + 400}8 = 50\sqrt{3} + 50$ .

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 $875$ .

Solution 3

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.

2007 AIME I (Problems • Answer Key • Resources)
Preceded by Problem 11	Followed by Problem 13
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