vì a;b;c là độ dài 3 cạnh của 1 tam giác áp dụng bđt tam giác ta có\(\Rightarrow\hept{\begin{cases}a+b>c\Rightarrow a+b-c>0\\a+c>b\Rightarrow a+c-b>0\\b+c>a\Rightarrow b+c-a>0\end{cases}}\)

\(\Rightarrow\sqrt{a+b-c};\sqrt{a+c-b};\sqrt{b+c-a}\)luôn được xác định\(\left(\sqrt{a+b-c}-\sqrt{a+c-b}\right)>=0\Rightarrow a+b-c-2\sqrt{\left(a+b-c\right)\left(a+c-b\right)}+a+c-b\)\(>=0\Rightarrow a+b-c+a+c-b>=2\sqrt{\left(a+b-c\right)\left(a+c-b\right)}\Rightarrow\frac{a+b-c+a+c-b}{2}=\frac{2a}{2}\)

\(=a>=\sqrt{\left(a+b-c\right)\left(a+c-b\right)}\)

tương tự ta có :\(b>=\sqrt{\left(a+b-c\right)\left(b+c-a\right)};c>=\sqrt{\left(a+c-b\right)\left(b+c-a\right)}\)

\(\Rightarrow abc>=\sqrt{\left(a+b-c\right)^2\left(a+c-b\right)^2\left(b+c-a\right)^2}=\left(a+b-c\right)\left(a+c-b\right)\left(b+c-a\right)\)

dấu = xảy ra khi a=b=c

dòng 3 là vì  \(\left(\sqrt{a+b-c}-\sqrt{a+c-b}\right)^2>=0\)nhá

Đặt \(\hept{\begin{cases}x=b+c-a\\y=a+c-b\\z=a+b-c\end{cases}}\left(x;y;z>0\right)\).Ta có:

\(x+y=b+c-a+a+c-b=2c\Rightarrow c=\frac{x+y}{2}\)

\(y+z=a+c-b+a+b-c=2a\Rightarrow a=\frac{y+z}{2}\)

\(z+x=a+b-c+b+c-a=2b\Rightarrow b=\frac{z+x}{2}\)

Do đó: \(A=\frac{y+z}{2x}+\frac{x+z}{2y}+\frac{x+y}{2z}\)

\(\Leftrightarrow2A=\left(\frac{x}{y}+\frac{y}{x}\right)+\left(\frac{y}{z}+\frac{z}{y}\right)+\left(\frac{z}{x}+\frac{x}{z}\right)\ge6\) (BĐT AM-GM)

\(\Rightarrow A\ge\frac{6}{2}=3\).Dấu "=" khi a=b=c

trước hết theo bđt tam giác chỉ ra được rằng \(\dfrac{a}{b+c-a};\dfrac{b}{a+c-b};\dfrac{c}{a+b-c}>0\)

áp dụng bất đẳng thức Cauchy-Schwarz:

\(A=\dfrac{a}{b+c-a}+\dfrac{b}{a+c-b}+\dfrac{c}{a+b-c}\)

\(A=\dfrac{a^2}{ab+ac-a^2}+\dfrac{b^2}{ab+bc-b^2}+\dfrac{c^2}{ac+bc-c^2}\)

\(A\ge\dfrac{\left(a+b+c\right)^2}{2\left(ab+bc+ac\right)-a^2-b^2-c^2}\)

Áp dụng bất đẳng thức AM-GM:

\(2\left(ab+bc+ac\right)-\left(a^2+b^2+c^2\right)\)

\(\le2\left(ab+bc+ac\right)-\left(ab+bc+ac\right)\)

\(=ab+bc+ac\)

Mặt khác,theo AM-GM: \(\left(a+b+c\right)^2\ge3\left(ab+bc+ac\right)\)

Hay: \(A\ge\dfrac{\left(a+b+c\right)^2}{2\left(ab+bc+ac\right)-a^2-b^2-c^2}\ge\dfrac{3\left(ab+bc+ac\right)}{ab+bc+ac}=3\)

Đặt \(b+c-a=x,a+c-b=y,a+b-c=z\)

\(\left\{{}\begin{matrix}x+y=b+c-a+a+c-b=2c\\y+z=a+c-b+a+b-c=2a\\x+z=b+c-a+a+b-c=2b\end{matrix}\right.\)

Có

\(A=\dfrac{a}{b+c-a}+\dfrac{b}{a+c-b}+\dfrac{c}{a+b-c}\)

\(\Leftrightarrow2A=\dfrac{2a}{b+c-a}+\dfrac{2b}{a+c-b}+\dfrac{2c}{a+b-c}\)

\(\Leftrightarrow2A=\dfrac{y+z}{x}+\dfrac{x+z}{y}+\dfrac{x+y}{z}\ge6\)

\(\Leftrightarrow2A=\left(\dfrac{y}{x}+\dfrac{x}{y}\right)+\left(\dfrac{y}{z}+\dfrac{z}{y}\right)+\left(\dfrac{x}{z}+\dfrac{z}{x}\right)\)

Ápdụng bất đẳng thức \(\dfrac{a}{b}+\dfrac{b}{a}\ge2\forall a,b>0\)

\(\Rightarrow2A\ge6\)

\(\Rightarrow A\ge3\left(đpcm\right)\)