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2022 AMC 12A Problem 8

Problem 8 of 25EasierAlgebraArithmetic

The infinite product 103⋅1033⋅10333⋯\sqrt[3]{10}\cdot\sqrt[3]{\sqrt[3]{10}}\cdot\sqrt[3]{\sqrt[3]{\sqrt[3]{10}}}\cdots evaluates to a real number. What is that number?

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Solution

The kkth factor is 1010 raised to the kk-fold cube root, namely 1013k.10^{\frac{1}{3^k}}. The product is 1010 raised to 13+19+127+⋯=131−13=12. \begin{aligned} &\frac13+\frac19+\frac1{27}+\cdots=\frac{\frac{1}{3}}{1-\frac{1}{3}} \\ &=\frac12. \end{aligned} So the value is 1012=10.10^{\frac{1}{2}}=\sqrt{10}. Thus, the correct answer is A.
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