Tricky fraction

1 2016 > m n > 1 2017 \large \frac{1}{2016}>\frac{m}{n}>\frac{1}{2017}

If m m and n n are positive integers satisfying the inequality above, find the minimum possible value of m + n m+n .


The answer is 4035.

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3 solutions

Kushal Bose
Aug 24, 2016

1 2016 > m n > 1 2017 \frac{1}{2016}>\frac{m}{n}>\frac{1}{2017}

We have to take minimum values.So multiplying 2 2 in neumerator and denominator :

2 4032 > m n > 2 4034 \frac{2}{4032}>\frac{m}{n}>\frac{2}{4034}

The smallest m n \frac{m}{n} is 2 4033 \dfrac{2}{4033}

how do you know that there can't be any solution less than 2 + 4033 = 4035 2 + 4033 = 4035 ?

Pi Han Goh - 4 years, 9 months ago

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I have multiplied the smallest number 2 in both neumerator and denominator and fixing the neumirator at 2 and variying the denominator.Now 2033 is the only fraction you can find within this limit.

Kushal Bose - 4 years, 9 months ago
Prince Loomba
Sep 3, 2016

The simplest way is bring n to numerator, the first term should be greater than 2, middle 2 and first less than 2. So n=4033 and m=2 satisfy minimum...

Sharky Kesa
Aug 26, 2016

In general, we can use the Farey Sequence to solve this question. This question is equivalent to finding the smallest denominator between the two fractions (since the numerator would be minimised as well, so their sum is minimised). Note that if a d b c = 1 |ad-bc|=1 , then, these two terms are adjacent terms in the Farey sequence. Thus, the fraction with the smallest denominator between them is

a b < a + c b + d < c d \dfrac {a}{b} < \dfrac {a+c}{b+d} < \dfrac {c}{d}

Therefore, if a d b c = 1 |ad-bc=1| , then the fraction with the smallest denominator between them is a + c b + d \dfrac {a+c}{b+d} , so the general sum is a + b + c + d a+b+c+d . Substituting the values is the question, we find that the if condition is satisfied, so the answer is 4035.

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