Egyptian Fractions?

Find the total number of unordered triplets of positive integers ( a , b , c ) (a,b,c) satisfying 1 a + 1 b + 1 c = 1. \dfrac1a + \dfrac1b + \dfrac1c = 1.


The answer is 3.

This section requires Javascript.
You are seeing this because something didn't load right. We suggest you, (a) try refreshing the page, (b) enabling javascript if it is disabled on your browser and, finally, (c) loading the non-javascript version of this page . We're sorry about the hassle.

1 solution

Jordan Cahn
Dec 26, 2018

Note that if a , b , c > 3 a,b,c>3 the given sum is impossible. Similarly, a , b , c > 1 a,b,c>1 . Thus, at least one of our integers must be 2 2 or 3 3 . WLOG, say it is a a . Then we have the following cases.

  • a = 2 a=2 . Then 1 b + 1 c = 1 2 \frac{1}{b} + \frac{1}{c} = \frac{1}{2} . We know that b > 1 b>1 . If b = 2 b=2 then 1 c = 0 \frac{1}{c}=0 , a contradiction. If b = 3 b=3 then c = 6 c=6 (first solution). If b = 4 b=4 then c = 4 c=4 (second solution). If b 5 b\geq 5 then 1 c 3 10 c 3 \frac{1}{c} \geq \frac{3}{10} \implies c \leq 3 . The only integers meeting this criteria are 3 3 and 2 2 , which we have already considered.

  • a = 3 a=3 . Then 1 b + 1 c = 2 3 \frac{1}{b} + \frac{1}{c} = \frac{2}{3} . We now consider possible b > 2 b>2 (if b = 2 b=2 then we are in the first case). If b = 3 b=3 then c = 3 c=3 (third solution). If b 4 b\geq 4 then 1 c 5 12 c 2 \frac{1}{c} \geq \frac{5}{12} \implies c \leq 2 . Again, we have already considered these cases.

Thus there are 3 \boxed{3} solutions.

0 pending reports

×

Problem Loading...

Note Loading...

Set Loading...