[tex]P^2=2+2(\sqrt{a^2+ab+bc+ca}+\sqrt{b^2+ab+bc+ca}+\sqrt{c^2+ab+bc+ca})\geq 2+2(\sqrt{a^2}+\sqrt{b^2}+\sqrt{c^2})=2+2(a+b+c)=4\Rightarrow P\geq 2[/tex]
Dấu "=" xảy ra khi a=b=0,c=1.
[tex]P^2=2+2(\sqrt{a^2+ab+bc+ca}+\sqrt{b^2+ab+bc+ca}+\sqrt{c^2+ab+bc+ca})\geq 2+2(\sqrt{a^2}+\sqrt{b^2}+\sqrt{c^2})=2+2(a+b+c)=4\Rightarrow P\geq 2[/tex]
Dấu "=" xảy ra khi a=b=0,c=1.