Symmetric Equations

Find four distinct positive integers a , b , c , d a, b, c, d such that:

a b = c + d ab=c+d and c d = a + b cd=a+b

Submit your answer as the sum of the squares of a , b , c , d a, b, c, d .


The answer is 39.

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2 solutions

Scrub Lord
Feb 4, 2018

If a>2 and b>2 and c>2 and d>2, then a + b < ab = c + d < cd, which is a contradiction. Therefore, WLOG we can assume that d is either 1 or 2. d = 1:

ab = a + b + 1

ab - a - b + 1 = 2

(a - 1)(b - 1) = 2, a = 3 and b = 2.

d = 2:

ab = a/2 + b/2 + 2

ab - a/2 - b/2 + 1/4 = 9/4

(2a - 1)(2b - 1) = 9

This holds no new distinct solutions for a and b.

Vladimir Smith
Mar 13, 2016

a = 5 , b = 1 , c = 2 , d = 3 a=5, b=1, c=2, d=3 Or some other arrangement of the same numbers.

a=-4 ,b=0 ,c=-2 and d=2 are also a arrangement bcz here no discuss about positive integer

shyam upadhyay - 5 years, 3 months ago

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Oops, thanks for noticing and posting it here, fixed now.

Vladimir Smith - 5 years, 3 months ago

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