Maths Olympiad Prep

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Algebra Difficulty 5.0 AIME, harder Prove it JBMO

Problem:
For any real number aa, let a\lfloor a\rfloor denote the greatest integer not exceeding aa. In positive real numbers solve the following equation
n+n+n3=2014 n+\lfloor\sqrt{n}\rfloor+\lfloor\sqrt[3]{n}\rfloor=2014

Solutions — 2

Solution 1

Solution:
Obviously nn must be a positive integer. Now note that 442=1936<2014<2025=45244^2=1936<2014<2025=45^2 and 123<1900<2014<13312^3<1900<2014<13^3.
If n<1950n<1950 then 2014=n+n+n3<1950+44+12=20062014=n+\lfloor\sqrt{n}\rfloor+\lfloor\sqrt[3]{n}\rfloor<1950+44+12=2006, a contradiction!
So n1950n \geq 1950. Also if n>2000n>2000 then 2014=n+n+n3>2000+44+12=20562014=n+\lfloor\sqrt{n}\rfloor+\lfloor\sqrt[3]{n}\rfloor>2000+44+12=2056, a contradiction!
So 1950n20001950 \leq n \leq 2000, therefore n=44\lfloor\sqrt{n}\rfloor=44 and n3=12\lfloor\sqrt[3]{n}\rfloor=12. Plugging that into the original equation we get:
n+n+n3=n+44+12=2014 n+\lfloor\sqrt{n}\rfloor+\lfloor\sqrt[3]{n}\rfloor=n+44+12=2014
From which we get n=1958n=1958.

Solution 2

Solution:
Obviously nn must be a positive integer. Since n2014n \leq 2014, n<45\sqrt{n}<45 and n3<13\sqrt[3]{n}<13.
From n=2014nn3>20144513=1956n=2014-\lfloor\sqrt{n}\rfloor-\lfloor\sqrt[3]{n}\rfloor>2014-45-13=1956, n>44\sqrt{n}>44 and n3>12\sqrt[3]{n}>12, thus n=44\lfloor\sqrt{n}\rfloor=44 and n3=12\lfloor\sqrt[3]{n}\rfloor=12 and n=2014nn3=20144412=1958n=2014-\lfloor\sqrt{n}\rfloor-\lfloor\sqrt[3]{n}\rfloor=2014-44-12=1958.

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