Maths Olympiad Prep

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Number theory Difficulty 5.3 AIME, harder Find the answer

The first five decimal places of the square root of a natural number are 44994, and the first five decimal places of the square root of the number that is 2 greater are 49444. What are these numbers?

A number or a short expression. Spacing and $ signs are ignored.

Solution

Let the two integers in question be nn and n+2n+2. The integer parts of their square roots must be the same, otherwise the difference between the two square roots would be more than 1, but

n+2n=2n+2+n10.04461>10.0451=1919>21 \sqrt{n+2}-\sqrt{n}=\frac{2}{\sqrt{n+2}+\sqrt{n}}\frac{1}{0.0446}-1>\frac{1}{0.045}-1=\frac{191}{9}>21

Since AA is an integer, only 22 can fit. Then nn is the integer between 22.44222.44^2 and 22.45222.45^2, that is,

503.5536<n<504.0025 503.5536<n<504.0025

so n=504n=504, and indeed

504=22.449944506=22.494443 \begin{aligned} & \sqrt{504}=22.449944 \ldots \\ & \sqrt{506}=22.494443 \ldots \end{aligned}

Agnes Baranyai (Budapest, Martos F. Secondary School II. grade)

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Source: NuminaMath-1.5, licensed Apache-2.0. Statement and solution reproduced as published; topic and difficulty added by this site.