You Can't Get An Equation Shorter Than This.

Algebra Level 5

If the sum of all real solutions of the equation : { x } x = 2014 x \left\{x\right\}\left\lfloor x\right\rfloor =2014 x can be written as a b \frac{-a}{b} where a , b a,b are coprime positive integers, find a + b a+b .


Details and assumption :

\left\lfloor \cdot\right\rfloor is the floor function (the greatest integer function) and { x } = x x \{x\}=x-\left\lfloor x\right\rfloor .


The answer is 2016.

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4 solutions

Patrick Corn
Aug 27, 2014

Write x = n + r x = n+r with n Z n \in \mathbb Z and 0 r < 1 0 \le r < 1 . Then r n = 2014 ( r + n ) , rn = 2014(r+n), so ( 2014 r ) ( 2014 n ) = 201 4 2 (2014-r)(2014-n) = 2014^2 , so 2014 n 2014 - n is between 2014 2014 and 201 4 2 / 2013 2014^2/2013 , so n n is between 2014 / 2013 -2014/2013 and 0 0 . So n = 1 n = -1 or n = 0 n = 0 .

If n = 0 n = 0 then x = 0 x = 0 . If n = 1 n = -1 then r = 2014 / 2015 r = 2014/2015 , so x = 1 / 2015 x = -1/2015 . So the sum of these two solutions is 1 / 2015 -1/2015 , so the answer is 2016 \fbox{2016} .

Awesome solution.

Jayakumar Krishnan - 6 years, 9 months ago

Very imaginative and conclusive.

Lu Chee Ket - 6 years, 9 months ago
Danny He
Sep 3, 2014

Firstly, x = 0 x=0 is clearly a solution to the equation.

Now let x = y \lfloor x \rfloor = y and { x } = z \left\{x\right\} = z

So x = y + z x = y+z

y z = 2014 x = 2014 ( y + z ) yz = 2014x = 2014\left(y+z\right)

y z = 2014 y + 2014 z \Rightarrow yz = 2014y + 2014z

y z 2014 z = 2014 y yz - 2014z = 2014y

z = 2014 y y 2014 z = \frac{2014y}{y-2014}

0 z < 1 0 2014 y y 2014 < 1 0 \leq z < 1 \Rightarrow 0 \leq \frac{2014y}{y-2014} < 1

Note also that if we assume x > 0 x>0 , then y < x y < x and z < 1 z<1 so y z < x y z 2014 x yz < x \Rightarrow yz \neq 2014x

This means that if there is indeed a non-zero solution to the equation, then x x cannot be positive, and thus x , y < 0 x,y <0

Now we see that y 2014 < 0 y 2014 < 2014 y 0 y-2014 < 0 \Rightarrow y-2014 < 2014y \leq 0

Taking the left half we see that 2013 y < 2014 y > 2014 2013 -2013y < 2014 \Rightarrow y> \frac{-2014}{2013}

0 > y > 2014 2013 \Rightarrow 0 > y > \frac{-2014}{2013}

y Z y = 1 y \in \mathbb{Z} \Rightarrow y = -1

y z = 2014 y + 2014 z y = 2014 z z 2014 yz = 2014y + 2014z \Rightarrow y = \frac{2014z}{z-2014}

2014 z = 2014 z z = 2014 2015 2014-z = 2014z \Rightarrow z = \frac{2014}{2015}

So the sum of solutions is then 0 + ( y + z ) = 0 + 1 + 2014 2015 = 1 2015 0 + \left(y+z\right) = 0 + -1 + \frac{2014}{2015} = \frac{1}{2015}

1 + 2015 = 2016 1 + 2015 = 2016

Michal Habera
Sep 1, 2014

Let x : = m + ε x:=m + \varepsilon where m Z m \in \mathbb{Z} and ε 0 , 1 ) \varepsilon \in \langle 0,1) . Thus m ε = 2014 ( m + ε ) m\varepsilon = 2014(m+\varepsilon) . Let express ε = ε ( m ) = 2014 m m 2014 \varepsilon = \varepsilon(m) = \frac{2014m}{m-2014} , epsilon as function of m. To satisfy assumption ε 0 , 1 ) \varepsilon \in \langle 0,1) :

ε ( 0 ) = 0 \varepsilon(0) = 0 thus x 1 = 0 x_1 = 0 .

ε ( 1 ) = 2014 2013 < 0 \varepsilon(1) = -\frac{2014}{2013} < 0 , for any m > 1 |m| > 1 : ε \varepsilon is either negative or bigger than 1 1 (numerator increases linearly with coeff 2014 2014 , denominator linearly with coeff 1 1 ).

ε ( 1 ) = 2014 2015 \varepsilon(-1) = \frac{2014}{2015} thus x 2 = 1 + 2014 2015 = 1 2015 x_2 = -1 + \frac{2014}{2015} = -\frac{1}{2015} and a + b = 2016 . \boxed{ a+b = 2016}.

Lu Chee Ket
Sep 1, 2014

Begin from the end:

(P, Q) = (0, 0) and (- 0, - 4.962779156327543424317617866005e-4)

x1 = 0

x2 = - 4.962779156327543424317617866005e-4

x1 + x2 = - 4.962779156327543424317617866005e-4 = - 1/ 2015 = - a/ b

a + b = 2016

Check with x = -1/ 2015,

(x - Floor(x)) Floor(x) = 2014 x

(-1/ 2015 + 1)(-1) = 2014 (-1/ 2015)

-0.9995037220843672456575682382134 =

-0.9995037220843672456575682382134

Working:

Int p * Frac p = 2014 (Int p + Frac p) ELSE (Int -p - 1)(Frac -p + 1) = 2014 (Int -p + Frac -p)

P Q = 2014 (P + Q) OR (- P - 1)(- Q + 1) = - 2014 (P + Q)

Q = 2014 + 2014^2/ (P - 2014) OR Q = - 2013 + (2013*2015 + 1)/ (P + 2015)

Search with positive integers P from 0 to extreme only, for Q > 0 and ABS(Q) < 1 as (P + Q) or -(P + Q) are combination of whole number and fractional decimals only.

Only (0, 0) and (0, - 1/ 2015) are solutions.

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