Maths Olympiad Prep

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, 2022

Geometry Difficulty 5.5 AIME, harder Find the answer United States

Problem:

Parallel lines 1\ell_{1}, 2\ell_{2}, 3\ell_{3}, 4\ell_{4} are evenly spaced in the plane, in that order. Square ABCDABCD has the property that AA lies on 1\ell_{1} and CC lies on 4\ell_{4}. Let PP be a uniformly random point in the interior of ABCDABCD and let QQ be a uniformly random point on the perimeter of ABCDABCD. Given that the probability that PP lies between 2\ell_{2} and 3\ell_{3} is 53100\frac{53}{100}, the probability that QQ lies between 2\ell_{2} and 3\ell_{3} can be expressed as ab\frac{a}{b}, where aa and bb are relatively prime positive integers. Compute 100a+b100a + b.

A number or a short expression. Fractions can be typed as 3/2, and spacing doesn't matter.

Solution

Solution:

The first thing to note is that the area of ABCDABCD does not matter in this problem, so for the sake of convenience, introduce coordinates so that A=(0,0)A = (0, 0), B=(1,0)B = (1, 0), and C=(0,1)C = (0, 1).

Suppose AA and BB lie on the same side of 2\ell_{2}. Then, by symmetry, CC and DD lie on the same side of 3\ell_{3}. Now suppose BCBC intersects 2\ell_{2} and 3\ell_{3} at XX and YY, respectively, and that DADA intersects 2\ell_{2} and 3\ell_{3} at UU and VV, respectively. Note that XYVUXYVU is a parallelogram. Since BC=BX+XY+YC=BX+2XY>2XYBC = BX + XY + YC = BX + 2XY > 2XY, we have that XYXY is less than half the side length of the square, so the area of XYVUXYVU is at most half of the area of square ABCDABCD. However, since 0.53>120.53 > \frac{1}{2}, this can't happen. Similar reasoning applies if BB and CC lie on the same side of 3\ell_{3}. Therefore, points BB and DD lie between 2\ell_{2} and 3\ell_{3}.

Let ABAB and ADAD intersect 2\ell_{2} at points MM and NN, respectively. Let r=AMr = AM and s=ANs = AN. By symmetry, [AMN]=0.235[AMN] = 0.235, so rs=0.47rs = 0.47. Additionally, in coordinates line 2\ell_{2} is just xr+ys=1\frac{x}{r} + \frac{y}{s} = 1. Therefore line 4\ell_{4} is given by xr+ys=3\frac{x}{r} + \frac{y}{s} = 3. Since C=(1,1)C = (1, 1) lies on this line, 1r+1s=3\frac{1}{r} + \frac{1}{s} = 3.

The answer that we want is
12r+2s4=1r+s2 1 - \frac{2r + 2s}{4} = 1 - \frac{r + s}{2}
On the other hand, the condition 1r+1s=3\frac{1}{r} + \frac{1}{s} = 3 rearranges to 3rs=r+s3rs = r + s, so r+s=1.41r + s = 1.41. Thus the answer is 11.412=0.295=592001 - \frac{1.41}{2} = 0.295 = \frac{59}{200}.

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Source: MathNet, licensed CC-BY-4.0. Statement reproduced verbatim; metadata (topic, difficulty) added by this project.