Floor and fractional part functions

Algebra Level 3

Find the minimum value of real number x x such that: x × { x } < x 1 \large \lfloor x \rfloor \times \lbrace x \rbrace < x-1


The answer is 2.

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1 solution

Anirudh Sreekumar
Feb 15, 2018

x × { x } < x 1 x × { x } < ( x + { x } ) 1 Since x = x + { x } 1 x { x } + x × { x } < 0 ( x 1 ) ( { x } 1 ) < 0 ( 1 ) We have , 0 { x } < 1 1 { x } 1 < 0 ( 2 ) From ( 1 ) and ( 2 ) we have, ( x 1 ) > 0 x > 1 x 2 Thus the minimum value for the inequality to be satisfied occurs at x = 2 \begin{aligned}\lfloor x \rfloor \times \lbrace x \rbrace &< x-1\\ \lfloor x \rfloor \times \lbrace x \rbrace &< (\lfloor x \rfloor + \lbrace x \rbrace)-1\hspace{4mm}\small\color{#3D99F6}\text{Since } x=\lfloor x \rfloor + \lbrace x \rbrace\\ 1-\lfloor x \rfloor-\lbrace x \rbrace+\lfloor x \rfloor \times \lbrace x \rbrace&<0\\ \implies (\lfloor x\rfloor-1)(\lbrace x \rbrace-1)&<0\hspace{4mm}\color{#3D99F6}\small(1)\\\\ \text{We have ,}\\ 0\leq\lbrace x \rbrace&<1\\ \implies -1\leq\lbrace x \rbrace-1&<0\hspace{4mm}\color{#3D99F6}\small(2)\\\\ \text{From }\color{#3D99F6}\small(1) \color{#333333}\normalsize\text{ and }\color{#3D99F6}\small(2)\color{#333333}\normalsize\text{ we have,}\\ (\lfloor x\rfloor-1)&>0\\ \implies\lfloor x\rfloor&>1\\ \implies x&\geq2\\ \text{Thus the minimum value for the } &\text{inequality to be satisfied occurs at }x=\color{#EC7300}\boxed{\color{#333333}2}\end{aligned}

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