Second Problem for Floors

Algebra Level 4

Let a number x x be defined as follows:

x + x 4 + x 8 + x 16 + x 32 = 100. \large \lfloor\sqrt{x}\rfloor + \lfloor\sqrt[4]{x}\rfloor + \lfloor\sqrt[8]{x}\rfloor + \lfloor\sqrt[16]{x}\rfloor + \lfloor\sqrt[32]{x}\rfloor = 100 .

If the maximum value of x x can be expressed as c 2 1 c^2 - 1 , solve for c c .


For more problems like this, try answering this set .


The answer is 87.

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1 solution

Christian Daang
Jan 30, 2017

Since x = c 2 1 , x = c 1 x = c^2 - 1 , \implies \lfloor\sqrt{x}\rfloor = c - 1

c 1 + x 4 + x 8 + x 16 + x 32 = 100 x 4 + x 8 + x 16 + x 32 = 101 c Since 0 < c < 101 0 < c < 10 max ( c ) = 9 \large {\implies c - 1 + \lfloor\sqrt[4]{x}\rfloor + \lfloor\sqrt[8]{x}\rfloor + \lfloor\sqrt[16]{x}\rfloor + \lfloor\sqrt[32]{x}\rfloor = 100 \\ \lfloor\sqrt[4]{x}\rfloor + \lfloor\sqrt[8]{x}\rfloor + \lfloor\sqrt[16]{x}\rfloor + \lfloor\sqrt[32]{x}\rfloor = 101 - c \\ \text{Since} \ 0 < c < 101 \implies 0 < \lfloor\sqrt{c}\rfloor < 10 \\ \implies \max \ (\lfloor\sqrt{c}\rfloor) = 9}

x 4 = c 2 1 4 = c = 9 x 8 = c 2 1 8 = c 4 = c = 3 x 16 = c 2 1 16 = c 8 = c 4 = 1 x 32 = c 2 1 32 = c 16 = c 8 = 1 \large {\implies \lfloor\sqrt[4]{x}\rfloor = \lfloor\sqrt[4]{c^2 - 1}\rfloor = \lfloor\sqrt{c}\rfloor = 9 \\ \lfloor\sqrt[8]{x}\rfloor = \lfloor\sqrt[8]{c^2 - 1}\rfloor = \lfloor\sqrt[4]{c}\rfloor = \lfloor\sqrt{\sqrt{c}}\rfloor = 3 \\ \lfloor\sqrt[16]{x}\rfloor = \lfloor\sqrt[16]{c^2 - 1}\rfloor = \lfloor\sqrt[8]{c}\rfloor = \lfloor\sqrt{\sqrt[4]{c}}\rfloor = 1 \\ \lfloor\sqrt[32]{x}\rfloor = \lfloor\sqrt[32]{c^2 - 1}\rfloor = \lfloor\sqrt[16]{c}\rfloor = \lfloor\sqrt{\sqrt[8]{c}}\rfloor = 1 }

Hence,

x + x 4 + x 8 + x 16 + x 32 = c 1 + 9 + 3 + 1 + 1 = 100 c = 87 max (x) = 8 7 2 1. \large {\lfloor\sqrt{x}\rfloor + \lfloor\sqrt[4]{x}\rfloor + \lfloor\sqrt[8]{x}\rfloor + \lfloor\sqrt[16]{x}\rfloor + \lfloor\sqrt[32]{x}\rfloor = c - 1 + 9 + 3 + 1 + 1 = 100 \\ \boxed {c = 87} \implies \text{max (x)} = 87^2 - 1 .}

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