Giải:

Biến đổi vế trái, ta được:

(a−1)(b−1)(c−1)(a−1)(b−1)(c−1)

=(ab−a−b+1)(c−1)=(ab−a−b+1)(c−1)

=abc−ab−ac+a−bc+b+c−1=abc−ab−ac+a−bc+b+c−1

=abc−ab−ac−bc+a+b+c−1=abc−ab−ac−bc+a+b+c−1

=abc−(ab+ac+bc)+(a+b+c)−1=abc−(ab+ac+bc)+(a+b+c)−1

Thay ab + ac + bc = abc và a + b + c = 1, ta được:

=abc−abc+1−1=abc−abc+1−1

=0

 Ta có:

( a − 1 ) ( b − 1 ) ( c − 1 )

=( ab − a − b + 1) .( c − 1 )

=( abc − ab ) + ( −ac + a ) + ( −bc + b ) + ( c − 1 )

= abc − ( ab + bc + ca ) + ( a + b + c ) − 1

= [ abc − ( ab + bc +ca ) ] + [a + b + c − 1 ]

= 0 + 0

=0

Bài 2 : 

\(\left(a+b+c\right)^2=3\left(ab+bc+ca\right)\)

<=> a^2 + b^2 + c^2 + 2ab + 2bc + 2ca = 3ab + 3bc + 3ca 

<=> a^2 + b^2 + c^2 = ab + bc + ca 

<=> 2a^2 + 2b^2 + 2c^2 = 2ab + 2bc + 2ca 

<=> ( a - b )^2 + ( b - c )^2 + ( c - a )^2 = 0 

<=> a = b = c 

1.

\(\Leftrightarrow2a^2+2b^2+18=2ab+6a+6b\)

\(\Leftrightarrow\left(a^2-2ab+b^2\right)+\left(a^2-6a+9\right)+\left(b^2-6b+9\right)=0\)

\(\Leftrightarrow\left(a-b\right)^2+\left(a-3\right)^2+\left(b-3\right)^2=0\)

\(\Leftrightarrow\left\{{}\begin{matrix}a-b=0\\a-3=0\\b-3=0\end{matrix}\right.\) \(\Leftrightarrow a=b=3\)

2.

\(\left(a+b+c\right)^2=3\left(ab+bc+ca\right)\)

\(\Leftrightarrow a^2+b^2+c^2+2ab+2bc+2ca=3ab+3bc+3ca\)

\(\Leftrightarrow2a^2+2b^2+2c^2-2ab-2bc-2ca=0\)

\(\Leftrightarrow\left(a^2-2ab+b^2\right)+\left(b^2-2bc+c^2\right)+\left(c^2-2ca+a^2\right)=0\)

\(\Leftrightarrow\left(a-b\right)^2+\left(b-c\right)^2+\left(c-a\right)^2=0\)

\(\Leftrightarrow\left\{{}\begin{matrix}a-b=0\\b-c=0\\c-a=0\end{matrix}\right.\) \(\Leftrightarrow a=b=c\)

1)

Ta có: \(M=\Sigma_{cyc}\frac{\sqrt{3}\left(a+b+4c\right)}{\sqrt{3\left(a+b\right)\left(a+b+4c\right)}}\ge\Sigma_{cyc}\frac{\sqrt{3}\left(a+b+4c\right)}{\frac{3\left(a+b\right)+\left(a+b+4c\right)}{2}}=\Sigma_{cyc}\frac{\sqrt{3}\left(a+b+4c\right)}{2\left(a+b+c\right)}=3\sqrt{3}\)

Dấu "=" xảy ra khi a=b=c

2)

\(\Sigma_{cyc}\sqrt[3]{\left(\frac{2a}{ab+1}\right)^2}=\Sigma_{cyc}\frac{2a}{\sqrt[3]{2a\left(ab+1\right)^2}}\ge\Sigma_{cyc}\frac{2a}{\frac{2a+\left(ab+1\right)+\left(ab+1\right)}{3}}=3\Sigma_{cyc}\frac{a}{ab+a+1}\)

Ta có bổ đề: \(\frac{a}{ab+a+1}+\frac{b}{bc+b+1}+\frac{c}{ca+c+1}=1\left(abc=1\right)\)

\(\Rightarrow\Sigma_{cyc}\sqrt[3]{\left(\frac{2a}{ab+1}\right)^2}\ge3\)

\(a^2+b^2>=2ab\)

\(b^2+c^2>=2bc\)

\(a^2+c^2>=2ac\)

=> \(2\left(a^2+b^2+c^2\right)>=2\left(ab+bc+ac\right)\)DẤU '=' xảy ra khi a=b=c

Vì \(ab+bc+ac=3\)  =>   \(\frac{1}{a}+\frac{1}{b}+\frac{1}{c}=\frac{3}{abc}\)

Đặt \(\frac{1}{a}=x\):  \(\frac{1}{b}=y\):  \(\frac{1}{c}=z\)=> x+y+z=3xyz

Ta có   \(4\left(\frac{1}{x}+\frac{1}{y}+\frac{1}{z}\right)+\frac{1}{xyz}\ge13\)

AD BĐT  \(\frac{1}{a}+\frac{1}{b}+\frac{1}{c}\ge\frac{9}{a+b+c}\) dấu = khi a=b=c ta có 

  \(4\left(\frac{1}{x}+\frac{1}{y}+\frac{1}{z}\right)\ge\frac{36}{x+y+z}\)=\(\frac{36}{3xyz}=\frac{12}{xyz}\)

=> \(\frac{12}{xyz}+\frac{1}{xyz}\ge13\)

=>  \(\frac{13}{xyz}\ge13\)

mà \(3xyz=x+y+z\ge3\sqrt[3]{xyz}\)dấu = khi x=y=z 

=> xyz\(\le1\)

=> đpcm