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Wednesday, October 28, 2015

A beautiful solution to a math contest problem

I was recently involved in writing a math contest at my university. One of the problems I contributed was the following:

Let L be the line passing through points (\sqrt{2},0) and (0,\sqrt{2}) of the XY -plane. Let C_0 be the circle of radius 1 centered at (0, 0). Let C_1, C_2, C_3,\ldots be an infinite sequence of circles such that each C_i  is centered on the x-axis, each C_i is tangent to L and C_{i+1} is tangent to C_i for all i\geq 0. What is the total area of all these circles? 

The solution I had in mind was to find a recursive formula for the radius of C_i and then sum the areas as a geometric series. One student came up with an absolutely stunning method, worthy of Archimedes. Consider the trapezoid T_0 formed from the x-axis, the line L and the two vertical tangents to C_0. The area of the trapezoid is 2\sqrt{2} while the area of C_0 is \pi. So the ratio \lambda of the area of C_0 to the area of T_0 is \frac{\pi}{2\sqrt{2}}. Now observe that the ratio of the area of C_i to the area of the trapezoid T_i formed from the vertical tangents to C_i, the line L and the x-axis, is still \lambda by similarity. So \sum \mathrm{Area}(C_i)=\lambda \sum \mathrm{Area}(T_i).
The sum of the areas of T_i is just the area of an isosceles right triangle with side lengths 1+\sqrt{2}, i.e. \frac{(1+\sqrt{2})^2}{2}. So we get \frac{\pi}{2\sqrt{2}}\cdot \frac{(1+\sqrt{2})^2}{2}. Magic!

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