Floor Function Multiplied By Fractional Part

Algebra Level 5

Find the number of solutions for the equation { x } x = x \{ x\} \lfloor x \rfloor = x for x [ 100 , 100 ] x \in [-100,100] .

Details and assumptions :

{ x } \{ x\} denotes the fractional part of x x . x \lfloor x \rfloor denotes the greatest integer lesser than or equal to x x .


The answer is 101.

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3 solutions

Harsh Depal
Apr 3, 2014

{ x } [ x ] = x { x } [ x ] = [ x ] + { x } { x } = [ x ] [ x ] 1 A s { x } ( 0 , 1 ) 0 < [ x ] [ x ] 1 < 1 O n S o l v i n g T h e I n e q u a l i t y W e G e t , [ x ] 0 F o r E a c h [ x ] W e H a v e O n e { x } T h e r e f o r e E a c h [ x ] G i v e s O n e S o l u t i o n N u m b e r O f P o s s i b l e S o l u t i o n s I n G i v e n R a n g e I s 101 \{ x\} [x]=x\\ \{ x\} [x]=[x]+\{ x\} \\ \{ x\} =\frac { [x] }{ [x]-1 } \\ As\quad \{ x\} \quad \in \quad (0,1)\\ \quad 0<\frac { [x] }{ [x]-1 } <1\\ On\quad Solving\quad The\quad Inequality\quad We\quad Get,\\ [x]\quad \le 0\\ For\quad Each\quad [x]\quad We\quad Have\quad One\quad \{ x\} \\ Therefore\quad Each\quad [x]\quad Gives\quad One\\ Solution\\ \therefore \quad Number\quad Of\quad Possible\quad Solutions\quad In\quad Given\\ Range\quad Is\quad 101\\ \\

Boy, Check the range of {x} Its not (-1,1) But its (0,1)

Even though the answer is still 101

Vishal Sharma - 7 years, 2 months ago

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Thanks for informing

Harsh Depal - 7 years, 2 months ago

your question is wrongly phrased.... greatest integer greater than or equal to x is infinity.

pulkit gupta - 7 years ago
Sandeep Bhardwaj
Sep 15, 2014

By simply drawing graphs of y=x and y={x}[x]. It will have 101 solutions. I like solving such equations by graphical method. That one is cool.!!! try it , i am sure you will enjoy drawing the graphs of the two expressions on L.H.S and R.H.S.

I am using graphic method as Sandeep Bhardwaj has suggested. I plotted f ( x ) = { x } x f(x) = \{ x \} \lfloor x \rfloor with a spreadsheet.

It can be seen that for every negative number there is a solution to the equation. There is a trivial solution when x = 0 x=0 . When x > 0 x>0 , f ( x ) < 0 f(x)<0 and there is no solution. Therefore the number of solutions is 100 + 1 = 101 100+1=\boxed{101} .

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