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Geometry Difficulty 4.6 AIME Find the answer United States

Problem:

Square ABCDABCD is drawn. Isosceles triangle CDECDE is drawn with EE a right angle. Square DEFGDEFG is drawn. Isosceles triangle FGHFGH is drawn with HH a right angle. This process is repeated infinitely so that no two figures overlap each other. If square ABCDABCD has area 11, compute the area of the entire figure.

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

Solution

Solution:

Let the area of the nnth square drawn be SnS_{n} and the area of the nnth triangle be TnT_{n}. Since the hypotenuse of the nnth triangle is of length Sn\sqrt{S_{n}}, its legs are of length l=Sn2l=\sqrt{\frac{S_{n}}{2}}, so Sn+1=l2=Sn2S_{n+1}=l^{2}=\frac{S_{n}}{2} and Tn=l22=Sn4T_{n}=\frac{l^{2}}{2}=\frac{S_{n}}{4}. Using the recursion relations, Sn=12n1S_{n}=\frac{1}{2^{n-1}} and Tn=12n+1T_{n}=\frac{1}{2^{n+1}}, so Sn+Tn=12n1+12n+1=(12+2)12n=5212nS_{n}+T_{n}=\frac{1}{2^{n-1}}+\frac{1}{2^{n+1}}=\left(\frac{1}{2}+2\right) \frac{1}{2^{n}}=\frac{5}{2} \frac{1}{2^{n}}. Thus the total area of the figure is n=1Sn+Tn=52n=112n=52\sum_{n=1}^{\infty} S_{n}+T_{n}=\frac{5}{2} \sum_{n=1}^{\infty} \frac{1}{2^{n}}=\frac{5}{2}.

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