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1995 AIME Problems/Problem 1

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Problem

Square S_{1} is 1\times 1. For i\ge 1, the lengths of the sides of square S_{i+1} are half the lengths of the sides of square S_{i}, two adjacent sides of square S_{i} are perpendicular bisectors of two adjacent sides of square S_{i+1}, and the other two sides of square S_{i+1}, are the perpendicular bisectors of two adjacent sides of square S_{i+2}. The total area enclosed by at least one of S_{1}, S_{2}, S_{3}, S_{4}, S_{5} can be written in the form m/n, where m and n are relatively prime positive integers. Find m-n.

Image:AIME 1995 Problem 1.png

Solution

The sum of the areas of the squares if they were not interconnected is a geometric sequence:

1^2 + \left(\frac{1}{2}\right)^2 + \left(\frac{1}{4}\right)^2 + \left(\frac{1}{8}\right)^2 + \left(\frac{1}{16}\right)^2

Then subtract the areas of the intersections, which is \left(\frac{1}{4}\right)^2 + \ldots + \left(\frac{1}{32}\right)^2:

1^2 + \left(\frac{1}{2}\right)^2 + \left(\frac{1}{4}\right)^2 + \left(\frac{1}{8}\right)^2 + \left(\frac{1}{16}\right)^2 - \l...
= 1 + \frac{1}{2}^2 - \frac{1}{32}^2

The majority of the terms cancel, leaving 1 + \frac{1}{4} - \frac{1}{1024}, which simplifies down to \frac{1024 + \left(256 - 1\right)}{1024}. Thus, m-n =  255.

Alternatively, take the area of the first square and add \,\frac{3}{4} of the areas of the remaining squares. This results in 1+ \frac{3}{4}\left[\left(\frac{1}{2}\right)^2 + \ldots + \left(\frac{1}{16}^2\right)\right], which when simplified will produce the same answer.

See also

1995 AIME (ProblemsResources)
Preceded by
First Question
Followed by
Problem 2
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15
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