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smallest n fow which the radius of the n-th circle is an integer >1

Source: (2021-) 2022 XV 15th Dürer Math Competition Finals Day 2 E5

December 12, 2022
geometryareascircles

Problem Statement

Benedek draws circles with the same center in the following way. The first circle he draws has radius 11. Next, he draws a second circle such that the ring between the first and second circles has twice the area of the first circle. Next, he draws a third circle such that the ring between the second and third circles is three times the area of the first circle, and so on (see the diagram). What is the smallest nn fow which the radius of the nn-th circle is an integer greater than 11? https://cdn.artofproblemsolving.com/attachments/e/2/afa6d5ead6f2252aa821028370a3768912e674.png