Cover up some fractions!

Algebra Level 1

1 1 = 1 2 + 1 2 , 1 2 = 1 3 + 1 6 , 1 3 = 1 4 + 1 12 \frac11 = \frac12 + \frac12,\qquad \frac12 = \frac13 + \frac16,\qquad \frac13 = \frac14 + \frac1{12}

Is it true that for all positive integers N N , the value of 1 N \frac1N can be expressed as the sum of the reciprocals of 2 positive integers: 1 N = 1 A + 1 B ? \dfrac1N = \dfrac1A + \dfrac1B?

Yes No

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1 solution

David Vreken
Feb 25, 2018

Yes, because for every positive integer N N , at least one solution for 1 N = 1 A + 1 B \frac{1}{N} = \frac{1}{A} + \frac{1}{B} is A = N + 1 A = N + 1 and B = N ( N + 1 ) B = N(N + 1) , because 1 A + 1 B \frac{1}{A} + \frac{1}{B} = = 1 N + 1 + 1 N ( N + 1 ) \frac{1}{N + 1} + \frac{1}{N(N + 1)} = = N N ( N + 1 ) + 1 N ( N + 1 ) \frac{N}{N(N + 1)} + \frac{1}{N(N + 1)} = = N + 1 N ( N + 1 ) \frac{N + 1}{N(N + 1)} = = 1 N \frac{1}{N} .

If A and B don't have to be distinct (which is not a requirement here), then there is an even more trivial solution: A = B = 2N

1 2 N + 1 2 N = 2 2 N = 1 N \frac {1}{2N} + \frac {1}{2N} = \frac {2}{2N} = \frac {1}{N}

Zee Ell - 3 years, 3 months ago

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Yes, even better!

David Vreken - 3 years, 3 months ago

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