Three positive numbers are such that the sum of any one of them with the sum of squares of the remaining two numbers is the same. Is it true that all numbers are the same? (Author: L. Emelyanov)
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Tags: algebra proposed, algebra
modularmarc101
01.09.2011 05:23
Let these numbers be $a,b,c \in \mathbb{R}^+$, then $a+b^2+c^2 = a^2+b+c^2=a^2+b^2+c$. Considering these equalities two at a time, we get the following three equations:
$a+b^2=a^2+b \iff 0 = (a-b)(a+b-1)$
$b + c^2 = b^2+c \iff 0=(b-c)(b+c-1)$
$a^2+c=c^2+a \iff 0=(c-a)(c+a-1)$
If $a-b,b-c,c-a \neq 0$, then $a+b-1=b+c-1=c+a-1=0 \implies a=b=c=\frac{1}{2}$.
If two or more of $a-b,b-c,c-a$ are $0$, then it trivially follows that $a=b=c$.
However, if only one of $a-b,b-c,c-a$ is $0$, then we get infinitely many triples where all three numbers are not the same, such as those of the form $(a,b,c)=(t,t,1-t)$ with $0<t<\frac{1}{2}$ or $\frac{1}{2}<t<1$.
Therefore, $\boxed{\text{no}}$, not all numbers are always the same.
Mewto55555
01.09.2011 06:11
basketball9 wrote: So either $a=b$ or $a+b=1$. Assuming the $a/neqb$ we will have $a+b=1$ and similarly $b+c=1$ and $c+a=1$. This doesn't follow; you can have $a=b$ and $a+c=b+c=1$.