<s>[INST] Find the minimum value of $f(x,y,z)=\frac{x^2}{(x+y)(x+z)}+\frac{y^2}{(y+z)(y+x)}+\frac{z^2}{(z+x)(z+y)}$. Find the minimum value of $f(x,y,z)=\frac{x^2}{(x+y)(x+z)}+\frac{y^2}{(y+z)(y+x)}+\frac{z^2}{(z+x)(z+y)}$ for all notnegative value of $x,y,z$.
I think that minimum value is $\frac{3}{4}$ when $x=y=z$ but I have no prove.
 [/INST] By C-S $$\sum_{cyc}\frac{x^2}{(x+y)(x+z)}\geq\frac{(x+y+z)^2}{\sum\limits_{cyc}(x+y)(x+z)}=\frac{\sum\limits_{cyc}(x^2+2xy)}{\sum\limits_{cyc}(x^2+3xy)}\geq\frac{3}{4},$$
where the last inequality it's $$\sum_{cyc}(x-y)^2\geq0.$$
The equality occurs for $x=y=z$, which says that we got a minimal value.
Another way:
We need to prove that:
$$4\sum_{cyc}x^2(y+z)\geq3\prod_{cyc}(x+y)$$ or
$$\sum_{cyc}(x^2y+x^2z-2xyz)\geq0$$ or
$$\sum_{cyc}z(x-y)^2\geq0.$$
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