Find out the number of integers

Algebra Level 3

Find the number of real x x satisfying the equation { x } x 2 x = { x } 1 \{x\} \lfloor x \rfloor- 2\lfloor x \rfloor = \{x\} -1 .

Notations :


The answer is 0.

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3 solutions

Krish Shah
Apr 13, 2020

Let x = n ⌊x⌋ = n and { x } = r \{x\}\ = r . Then, applying Simon's favorite factoring trick,

n r 2 n = r 1 nr - 2n = r - 1

n ( r 2 ) r + 1 + ( 1 ) = 0 + ( 1 ) n(r-2) - r + 1 + (1) = 0 + (1)

n ( r 2 ) ( r 2 ) = 1 n(r-2) - (r-2) = 1

( n 1 ) ( r 2 ) = 1 (n-1)(r-2) = 1

Now, considering the range of r r , we know that 0 r < 1 0 ≤ r < 1 .

So, 2 r 2 < 1 -2 ≤ r-2 < -1 .

Multiplying the inequality by n 1 n-1 , with n 1 n≠1 , we get

2 ( n 1 ) ( r 2 ) ( n 1 ) < ( n 1 ) -2(n-1) ≤ (r-2)(n-1) < -(n-1)

Dealing with both sides of the inequality separately, we have the following for the first part of the inequality:

2 ( n 1 ) ( r 2 ) ( n 1 ) -2(n-1) ≤ (r-2)(n-1)

2 ( n 1 ) 1 -2(n-1) ≤ 1

2 ( n 1 ) 1 2(n-1) ≥ -1

( n 1 ) 1 / 2 (n-1) ≥ -1/2 or n 1 / 2 n ≥ 1/2

Now for the second part of the inequality:

1 < ( n 1 ) 1 < -(n-1)

1 > ( n 1 ) -1 > (n-1) or n < 0 n<0 , which is a contradiction to the previous result obtained.

As n n cannot be greater than 1 / 2 1/2 and less than 0 0 at the same time, we conclude there are no solutions for n n and so there are no solutions for x x .

Sohel Zibara
Jan 5, 2019

x integer --> {x} = 0 --> [x] = 1/2 impossible. thus no integer solutions.

x non-integer --> [x] = 1 - 1/(2 - {x}) which cannot be an integer since 2 - {x} € (1/2 , 1). thus no non-integer solutions.

No solutions

Aaghaz Mahajan
Mar 6, 2018

By simple manipulation we can write fractional part of x in terms of the floor function..........Then, applying the inequality of it being between 0 and 1, we can see that no solution exists....

Please give a bit more detail

Sajeev Yadav - 3 years, 1 month ago

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