ISL 2008 A2

by utkarshgupta, Feb 3, 2015, 5:01 PM

Problem :
(i) If $ x$, $ y$ and $ z$ are three real numbers, all different from $ 1$, such that $ xyz = 1$, then prove that
$ \frac {x^{2}}{\left(x - 1\right)^{2}} + \frac {y^{2}}{\left(y - 1\right)^{2}} + \frac {z^{2}}{\left(z - 1\right)^{2}} \geq 1$.
(With the $ \sum$ sign for cyclic summation, this inequality could be rewritten as $ \sum \frac {x^{2}}{\left(x - 1\right)^{2}} \geq 1$.)

(ii) Prove that equality is achieved for infinitely many triples of rational numbers $ x$, $ y$ and $ z$.

Author: Walther Janous, Austria

Solution :


\[\frac {x^{2}}{\left(x - 1\right)^{2}} + \frac {y^{2}}{\left(y - 1\right)^{2}} + \frac {z^{2}}{\left(z - 1\right)^{2}} \geq 1\]

$\iff \sum x^2(y-1)^2(z-1)^2 \ge (\prod(x-1))^2$

Setting $u=x+y+z$, $xy+yz+zx=v$ and $1=xyz=w$

\[\iff \sum x^2(y^2-2y+1)(z^2-2z+1) \ge (-x-y-z+xyz)^2\]

\[\iff \sum (1+x^2y^2+x^2z^2+x^2-2xy-2xz+2x+2x^2yz-2x^2y-2x^2z) \ge (u-v)^2\]

\[\iff 3+\sum x^2y^2 + \sum x^2 -4\sum xy +2 \sum x + 2\sum x - \sum x^2y+xy^2 \ge (v-u)^2\]

\[\iff 3+2(v^2-2uw)+(u^2-2v)-4v + 2u + 2u-\sum x^2y+xy^2 \ge (v-u)^2\]

\[\iff 3+2v^2-4u+u^2-2v-4v+4u \sum x^2y+xy^2 \ge (v-u)^2\]

\[\iff 3+v^2+2uv \ge 6v + 2\sum xy(x+y)\]

\[\iff 3+v^2+2uv \ge 6v + 2\sum xy(u-z)\]

\[\iff 3+v^2+2uv \ge 6v + 2uv - 6w\]

\[\iff (v-3)^2 \ge 0\]

which is obvious...

Equality holds when $ab+bc+ca=3$
Using this and $abc=1$,
It can be easily seen that there are infinitely many solutions.

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