The Floor Is Lava


Find all the real numbers $x$ such that $\lfloor x \lfloor x \rfloor \rfloor = 2024$.  

Note: $\lfloor x \rfloor$ denotes the highest integer less than or equal to $x$. For example, $\lfloor 2.5 \rfloor = 2$, $\lfloor \pi \rfloor = 3$, $\lfloor 24 \rfloor = 24$, $\lfloor 5.73 \rfloor = 5$, etc.



Proof Based Problems  


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Solution

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Solution for $x$ is $x\in (-45,-\frac{2024}{45}]$.

Proof:

Let $[x] = \lfloor x\rfloor$ and $\{x\} = x - [x] \implies [x] = x-\{x\}$

$\implies 2025 > [x]^2+[x]\{x\} \geq 2024$

Case 1: 

$[x]$ is positive.

$[x]$ is an integer and, $0\leq \{x\}<1 \implies 0\leq \{x\}[x]<[x]$

[[x[x]] = 2024 \implies [[x]^2+\{x\}[x]] = 2024\]

From the first part,

\[2025 >[x]^2+[x]\{x\}\implies 2025 > [x]^2 \implies [x]<45\]

From the second part,

\[[x]^2+[x]\{x\} \geq 2024\implies [x]^2 + [x] \geq 2024 \implies [x]>44\]

So, $45>[x]>44$, no integer solution for $[x]$


Case 2:

 $[x]$ is negative.

$[x]$ is an integer and $0\leq \{x\}<1 \implies 0\geq \{x\}[x]>[x]$

\[[x[x]] = 2024 \implies [[x]^2+\{x\}[x]] = 2024\]

From the first part,

\[2025 >[x]^2+[x]\{x\}\implies 2025 > [x]^2+[x] \implies [x]>-46\]

From the second part,

\[[x]^2+[x]\{x\} \geq 2024\implies [x]^2 \geq 2024 \implies [x]<-44\]

So, $-44>[x]>-46$ and so $[x] = -45$.

So, \[[-45x]=2024 \implies 2025 > -45x \geq 2024 \implies -\frac{2024}{45} \geq x > -45\]

So, the only solutions for $x$ is: $x\in (-45,-\frac{2024}{45}]$

This is a proof based problem added for learning purposes and does not accept submissions.

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