Can't Factor This Fractional Part

Algebra Level 5

Let x x be the smallest positive real satisfying { x 2 } 2 { x } + 1 = 0 \{x^2\}-2\{x\}+1=0

Find the value of 1000 { x } \lfloor 1000\{x\}\rfloor

Details and Assumptions

{ n } \{n\} is the fractional part of n n . That is, { n } = n n \{n\}=n-\lfloor n\rfloor for all positive real n n .


The answer is 732.

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1 solution

Bob Krueger
Jun 14, 2014

The trick here is that the square is inside of the fractional part so that we cannot easily factor the given expression. However, using the substitution found in the details and assumptions, we can get

{ x 2 } 2 { x } + 1 = 0 \left\{x^2\right\} - 2\left\{x\right\} + 1 = 0 x 2 x 2 2 x + 2 x + 1 = 0 \rightarrow x^2 - \left\lfloor x^2 \right\rfloor - 2x + 2\left\lfloor x \right\rfloor + 1 = 0 x 2 2 x + 1 = x 2 2 x \rightarrow x^2-2x+1 = \left\lfloor x^2 \right\rfloor - 2\left\lfloor x \right\rfloor ( x 1 ) 2 = x 2 2 x \rightarrow (x-1)^2 = \left\lfloor x^2 \right\rfloor - 2\left\lfloor x \right\rfloor

Since the right hand side of the above expression is an integer, the left hand side must also be an integer. Then x 1 = n x-1 = \sqrt{n} , for some nonnegative integer n n , so that we are guaranteed that the LHS is an integer. Since x x increases with n n , we can test successively larger values of n n until a solution is found. We only have to go to n = 3 n = 3 , and since this is the smallest solution, the answer is 1000 { 1 + 3 } = 732 \lfloor 1000\{ 1 + \sqrt{3} \} \rfloor = 732

Nicely done. I guess I shouldn't have mentioned that substitution in the Details and Assumptions, then? But it kinda is the definition of fractional part so... :P

Daniel Liu - 6 years, 12 months ago

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