If the sum of all real solutions of the equation : { x } ⌊ x ⌋ = 2 0 1 4 x can be written as b − a where a , b are coprime positive integers, find a + b .
Details and assumption :
⌊ ⋅ ⌋ is the floor function (the greatest integer function) and { x } = x − ⌊ x ⌋ .
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Awesome solution.
Very imaginative and conclusive.
Firstly, x = 0 is clearly a solution to the equation.
Now let ⌊ x ⌋ = y and { x } = z
So x = y + z
y z = 2 0 1 4 x = 2 0 1 4 ( y + z )
⇒ y z = 2 0 1 4 y + 2 0 1 4 z
y z − 2 0 1 4 z = 2 0 1 4 y
z = y − 2 0 1 4 2 0 1 4 y
0 ≤ z < 1 ⇒ 0 ≤ y − 2 0 1 4 2 0 1 4 y < 1
Note also that if we assume x > 0 , then y < x and z < 1 so y z < x ⇒ y z = 2 0 1 4 x
This means that if there is indeed a non-zero solution to the equation, then x cannot be positive, and thus x , y < 0
Now we see that y − 2 0 1 4 < 0 ⇒ y − 2 0 1 4 < 2 0 1 4 y ≤ 0
Taking the left half we see that − 2 0 1 3 y < 2 0 1 4 ⇒ y > 2 0 1 3 − 2 0 1 4
⇒ 0 > y > 2 0 1 3 − 2 0 1 4
y ∈ Z ⇒ y = − 1
y z = 2 0 1 4 y + 2 0 1 4 z ⇒ y = z − 2 0 1 4 2 0 1 4 z
2 0 1 4 − z = 2 0 1 4 z ⇒ z = 2 0 1 5 2 0 1 4
So the sum of solutions is then 0 + ( y + z ) = 0 + − 1 + 2 0 1 5 2 0 1 4 = 2 0 1 5 1
1 + 2 0 1 5 = 2 0 1 6
Let x : = m + ε where m ∈ Z and ε ∈ ⟨ 0 , 1 ) . Thus m ε = 2 0 1 4 ( m + ε ) . Let express ε = ε ( m ) = m − 2 0 1 4 2 0 1 4 m , epsilon as function of m. To satisfy assumption ε ∈ ⟨ 0 , 1 ) :
ε ( 0 ) = 0 thus x 1 = 0 .
ε ( 1 ) = − 2 0 1 3 2 0 1 4 < 0 , for any ∣ m ∣ > 1 : ε is either negative or bigger than 1 (numerator increases linearly with coeff 2 0 1 4 , denominator linearly with coeff 1 ).
ε ( − 1 ) = 2 0 1 5 2 0 1 4 thus x 2 = − 1 + 2 0 1 5 2 0 1 4 = − 2 0 1 5 1 and a + b = 2 0 1 6 .
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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Write x = n + r with n ∈ Z and 0 ≤ r < 1 . Then r n = 2 0 1 4 ( r + n ) , so ( 2 0 1 4 − r ) ( 2 0 1 4 − n ) = 2 0 1 4 2 , so 2 0 1 4 − n is between 2 0 1 4 and 2 0 1 4 2 / 2 0 1 3 , so n is between − 2 0 1 4 / 2 0 1 3 and 0 . So n = − 1 or n = 0 .
If n = 0 then x = 0 . If n = − 1 then r = 2 0 1 4 / 2 0 1 5 , so x = − 1 / 2 0 1 5 . So the sum of these two solutions is − 1 / 2 0 1 5 , so the answer is 2 0 1 6 .