FInd the sum of all positive integers x satisfying the equation
⌊ 2 0 x ⌋ + ⌊ 1 7 x ⌋ = 2 0 1 7
This problem is part of the set " Xenophobia "
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Sir apparently someone named "Rindell" shared your problem in our math group. Beware, he might be an impostor :p
For start, let us take the equation as simple fraction 2 0 x + 1 7 x = 2 0 1 7
upon solving we get x ≈ 1 8 , 5 3 4 . 5 9 5
if we put this value in original equation we get ⌊ 2 0 1 8 , 5 3 4 . 5 9 5 ⌋ + ⌊ 1 7 1 8 , 5 3 4 . 5 9 5 ⌋ = 2 0 1 6
which is one less than what is required. So, for increasing this value by one, we need a nearest number which is multiple of either 20 or 17, which in this case is 18,540. Now all values from 18,540 to 18,546 will satisfy the given equation as 18,547 is a multiple of 17, thereby will increase the value to 2018.
Finally sum of all values = 1 8 , 5 4 0 + 1 8 , 5 4 1 + ⋯ + 1 8 , 5 4 6 = 1 2 9 8 0 1
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Let y be a real number. Since ⌊ y ⌋ is the integer less than or equal to y , then y − 1 < ⌊ y ⌋ ≤ y .
In the same manner, 2 0 x + 1 7 x − 2 < ⌊ 2 0 x ⌋ + ⌊ 1 7 x ⌋ ≤ 2 0 x + 1 7 x
2 0 x + 1 7 x − 2 < 2 0 1 7 ≤ 2 0 x + 1 7 x
3 4 0 3 7 x − 2 < 2 0 1 7 ≤ 3 4 0 3 7 x
1 8 5 3 4 . 5 9 ≈ 2 0 1 7 × 3 7 3 4 0 ≤ x < 2 0 1 9 × 3 7 3 4 0 ≈ 1 8 5 5 2 . 9 7
1 8 5 3 5 ≤ x ≤ 1 8 5 5 2
Since ⌊ 2 0 1 8 5 3 5 ⌋ + ⌊ 1 7 1 8 5 3 5 ⌋ = 9 2 6 + 1 0 9 0 = 2 0 1 6 , we need to look at the next two integers which is a multiple of 2 0 or 1 7 which is 1 8 5 4 0 and 1 8 5 4 7 . Therefore the integral values of x that we need are 1 8 5 4 0 , 1 8 5 4 1 , … , 1 8 5 4 6 .
1 8 5 4 0 + 1 8 5 4 1 + … + 1 8 5 4 6 = 1 2 9 8 0 1