a b c, it's as easy as 1 2 3

Suppose a a , b b , c c are positive integers with a < b < c a < b < c such that 1 a + 1 b + 1 c = 1. \frac 1a + \frac 1b + \frac 1c = 1. What is a + b + c ? a + b + c?


The answer is 11.

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1 solution

Zach Abueg
Jan 5, 2017

First note that we must have 1 a < 1 \frac 1a < 1 , so a > 1 a > 1 .

Since 1 a > 1 b > 1 c \frac 1a > \frac 1b > \frac 1c , we must also have 1 a > 1 3 \frac 1a > \frac 13 ; so a < 3. a < 3. Thus, a = 2 a = 2 .

Now 1 b + 1 c = 1 2 \frac 1b + \frac 1c = \frac 12 where 2 < b < c . 2 < b < c.

Similar to before, 1 b > 1 4 \frac 1b > \frac 14 , so b < 4. b < 4. Thus, b = 3. b = 3.

With a = 2 a = 2 and b = 3 b = 3 , we have 1 2 + 1 3 + 1 c = 1 , \frac 12 + \frac 13 + \frac 1c = 1, which is satisfied when c = 6. c = 6.

To conclude, a + b + c = 2 + 3 + 6 = 11. a + b + c = 2 + 3 + 6 = 11.

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