a) Theo bất đẳng thức tam giác ta có
\(\Rightarrow\left\{{}\begin{matrix}a< b+c\\b< c+a\\c< a+b\end{matrix}\right.\) (1)
Ta có \(a+b+c=2\)
\(\Rightarrow\left\{{}\begin{matrix}b+c=2-a\\a+b=2-c\\a+c=2-b\end{matrix}\right.\) (2)
Từ (1) và (2)
\(\Rightarrow\left\{{}\begin{matrix}a< 2-a\\b< 2-b\\c< 2-c\end{matrix}\right.\)\(\Rightarrow\left\{{}\begin{matrix}2a< 2\\2b< 2\\2c< 2\end{matrix}\right.\)
\(\Rightarrow\left\{{}\begin{matrix}a< 1\\b< 1\\c< 1\end{matrix}\right.\) ( đpcm )
b) Áp dụng bất đẳng thức Cauchy - Schwarz
\(\Rightarrow\left(a+b-c\right)\left(c+a-b\right)\le\left(\dfrac{2a}{2}\right)^2=a^2\)
Tượng tự ta có \(\left\{{}\begin{matrix}\left(a+b-c\right)\left(b+c-a\right)\le b^2\\\left(b+c-a\right)\left(c+a-b\right)\le c^2\end{matrix}\right.\)
\(\Rightarrow\left(abc\right)^2\ge\left[\left(a+b-c\right)\left(b+c-a\right)\left(c+a-b\right)\right]^2\)
\(\Rightarrow abc\ge\left(a+b-c\right)\left(b+c-a\right)\left(c+a-b\right)\)
\(\Leftrightarrow9abc\ge8\left(ab+bc+ca\right)-8\)
\(\Leftrightarrow9abc+4\left(a^2+b^2+c^2\right)\ge8\left(ab+bc+ca\right)+4\left(a^2+b^2+c^2\right)-8\)
\(\Leftrightarrow9abc+4\left(a^2+b^2+c^2\right)\ge4\left(a+b+c\right)^2-8\)
\(\Leftrightarrow9abc+4\left(a^2+b^2+c^2\right)\ge8\) ( đpcm )
Dấu " = " xảy ra khi \(a=b=c=\dfrac{2}{3}\)