Prove that $\sqrt{a^2+3b^2}+\sqrt{b^2+3c^2}+\sqrt{c^2+3a^2}\geq6$ if $(a+b+c)^2(a^2+b^2+c^2)=27$
There seems to be bugs in the segment after where I marked "***". Needed for check.
When I saw the form $\sqrt{a^2+3b^2}$ I thought of the absolute value of a complex number.
So let
$$u=a+\sqrt{3}bi$$
$$v=b+\sqrt{3}ci$$
$$w=c+\sqrt{3}ai$$
And now what you want to prove becomes
$$|u|+|v|+|w|\geq6$$
$$u+v+w=(1+\sqrt{3}i)(a+b+c)$$
$$|u|^2+|v|^2+|w|^2=4(a^2+b^2+c^2)$$
$$(u+v+w)^2(|u|^2+|v|^2+|w|^2)$$
$$=(1+\sqrt{3}i)^2(a+b+c)^2\cdot4(a^2+b^2+c^2)$$
$$=4(1+\sqrt{3}i)^2(a+b+c)^2(a^2+b^2+c^2)$$
$$=4(1+\sqrt{3}i)^2\cdot27$$
$$|u+v+w|^2(|u|^2+|v|^2+|w|^2)=|4(1+\sqrt{3}i)^2\cdot27|=4\cdot27\cdot4$$
Now I thought I should separate $|u+v+w|$ to $|u|+|v|+|w|$ so that all the elements in the expression were independent $|u|$, $|v|$, $|w|$, and its form would be closer to the inequality we want to prove.
So I used the triangle inequality,
$$|u|+|v|+|w|\geq|u+v|+|w|\geq|u+v+w|$$
$$(|u|+|v|+|w|)^2(|u|^2+|v|^2+|w|^2)\geq|u+v+w|^2(|u|^2+|v|^2+|w|^2)=4\cdot27\cdot4$$
Let $x=|u|,\ \ y=|v|,\ \ z=|w|$
Then the problem becomes,
$$\mathrm{if\ \ }(x+y+z)^2(x^2+y^2+z^2)\geq4\cdot27\cdot4$$
$$\mathrm{prove\ that\ \ }x+y+z\geq6$$
Proof:
let $k$ be a number so that $x+y+z\geq k\geq0$ is always true.
A known formula:
$$(x-y)^2+(y-z)^2+(z-x)^2=3(x^2+y^2+z^2)-(x+y+z)^2\geq0$$
***Then
$$3(x^2+y^2+z^2)\geq(x+y+z)^2\geq k^2$$
$$\implies (x+y+z)^2(x^2+y^2+z^2)\geq k^2\cdot\frac{k^2}{3}=\frac{k^4}{3}$$
But it's already known that $(x+y+z)^2(x^2+y^2+z^2)\geq4\cdot27\cdot4$ has to be true. So to let $$(x+y+z)^2(x^2+y^2+z^2)\geq\frac{k^4}{3}$$ be always true,
$$\frac{k^4}{3}\leq4\cdot27\cdot4$$
$$k\leq6$$
This proof doesn't need $a,b,c\geq0$. It just needs them to be real numbers.