Find the distinct natural numbers $a,b,c,d$ who satisfying $ab=cd=a+b+c+d-3$.
Distinct natural numbers such that $ab=cd=a+b+c+d-3$
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calculus
real-analysis
number-theory
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2Please consider reading how to properly post homework question here http://meta.math.stackexchange.com/questions/1803/how-to-ask-a-homework-question – 2012-12-28
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1(2,6) and (3,4) are one possibility – 2012-12-28
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0$(1,1,1,1)$, $(3,3,3,3)$, $(2,6,3,4)$ are some examples. – 2012-12-28
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0What about playing around with the divisors of an abundant number ? – 2012-12-28
2 Answers
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Assume $a$ is the largest number among $a,b,c,d$; then $(a-1)b=a+c+d-3$
$$b=(a+c+d-3)/(a-1)<(a+a+a-3)/(a-1)=3$$ Hence, $b=1$ or $b=2$.
If $b=1$, then $a=a+1+c+d-3$. This implies $c+d=2$. Not the ideal pair.
If $b=2$, then $2a=a+2+c+d-3$. This implies $a=c+d-1$.
$$cd=ab=(c+d-1) \times 2$$ Hence, $$(c-2)(d-2)=2$$
Note that $c$ and $d$ are natural numbers.
Hence, $c-2 d-2$ is either $(-1,-2)$ or $(1,2)$. Only $(1,2)$ is the pair we want.
Hence, putting all this together, we get that $$a = 6, b=2, c=3, d= 4$$
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0distinct natural numbers means they are four different numbers? – 2012-12-31
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Find natural numbers $(a,b)$ such that:
$c$ = $\dfrac{1}{2}(a b-\sqrt{(a b-a-b+3)^2-4 a b)}-a-b+3 )$
and $d$ = $\dfrac{1}{2}$ $(a b+\sqrt{(a b-a-b+3)^2-4 a b)}-a-b+3)$
are natural numbers.