Ta có A=\(\left(ab+bc+ca\right)\left(\frac{1}{a}+\frac{1}{b}+\frac{1}{c}\right)-abc\left(\frac{1}{a^2}+\frac{1}{b^2}+\frac{1}{c^2}\right)\)

=\(2\left(a+b+c\right)+\frac{ab}{c}+\frac{bc}{a}+\frac{ca}{b}-\frac{ab}{c}-\frac{bc}{a}-\frac{ca}{b}=2\left(a+b+c\right)\)

\(A=\left(a+b\right)\left(a^2-ab+b^2\right)+3ab\left[\left(a+b\right)^2-2ab\right]+6a^2b^2=a^2-ab+b^2+3ab\left(1-2ab\right)+6a^2b^2\)

=\(\left(a+b\right)^2-3ab+3ab-6a^2b^2+6a^2b^2=1\)

2) Ta có \(A=\left(a-1\right)\left(b-1\right)\left(c-1\right)=abc-ab-bc-ca+a+b+c-1=0\)

Ta có:

$M=\dfrac{ab+bc+ca-a-b-c}{a^2b-a^2-b+1}$

Vì $abc=1$ nên:

$c=\dfrac1{ab}$

Suy ra: $M=\dfrac{ab+\dfrac1a+\dfrac1b-a-b-\dfrac1{ab}}{a^2b-a^2-b+1}$

$=\dfrac{(a-1)(b-1)(ab-1)}{ab(a^2-1)(b-1)}$

$=\dfrac{(a-1)(b-1)(ab-1)}{ab(a-1)(a+1)(b-1)}$

$=\dfrac{ab-1}{ab(a+1)}$

Mà $ab=\dfrac1c$ nên:

$M=\dfrac{\frac1c-1}{\frac1c(a+1)}$

$=\dfrac{1-c}{a+1}$

Vậy: $M=\dfrac{1-c}{a+1}$

1.

Cho $a+b+c=0$.

Ta có:

$a+b+c=0\Rightarrow a=-(b+c)$

$\Rightarrow a^2=(b+c)^2=b^2+c^2+2bc$

$\Rightarrow a^2-b^2-c^2=2bc$

Tương tự:

$b^2-c^2-a^2=2ca$

$c^2-a^2-b^2=2ab$

Do đó: $A=\dfrac{a^2}{2bc}+\dfrac{b^2}{2ca}+\dfrac{c^2}{2ab}$

$=\dfrac{a^3+b^3+c^3}{2abc}$

Mà $a+b+c=0$ nên:

$a^3+b^3+c^3=3abc$

Suy ra $A=\dfrac{3abc}{2abc}$

$A=\dfrac32$

2.

Cho $abc=2$.

Ta có:

$A=\dfrac{a}{ab+a+2}+\dfrac{b}{bc+b+1}+\dfrac{2c}{ac+2c+2}$

Vì $abc=2$ nên:

$ab=\dfrac2c,\quad bc=\dfrac2a,\quad ac=\dfrac2b$

Suy ra:

$A=\dfrac{ac}{2+ac+2c}+\dfrac{ab}{2+ab+a}+\dfrac{2bc}{2+2bc+2b}$

$=\dfrac{ac}{2+ac+2c}+\dfrac{ab}{2+ab+a}+\dfrac{bc}{1+bc+b}$

Quy đồng và sử dụng $abc=2$:

$A=1$

Bài 5.

1. Chứng minh

$\dfrac{2}{a}+\dfrac{1}{b}\ge\dfrac{4}{a+b}$

Ta có:

$\dfrac{2}{a}+\dfrac{1}{b}-\dfrac{4}{a+b}$

$=\dfrac{2b(a+b)+a(a+b)-4ab}{ab(a+b)}$

$=\dfrac{a^2-ab+2b^2}{ab(a+b)}$

$=\dfrac{(a-b)^2+b^2}{ab(a+b)}\ge0$

Vậy: $\boxed{\dfrac{2}{a}+\dfrac{1}{b}\ge\dfrac{4}{a+b}}$

2. Chứng minh

$\dfrac1a+\dfrac1b+\dfrac1c\ge\dfrac{a}{a+b+c}$

Vì $a,b,c>0$ nên:

$\dfrac1a+\dfrac1b+\dfrac1c>\dfrac1a$

Mà: $\dfrac1a>\dfrac{a}{a+b+c}$

Suy ra: $\boxed{\dfrac1a+\dfrac1b+\dfrac1c\ge\dfrac{a}{a+b+c}}$

1.

$a^3+b^4-ab(a+b)$

$=a^3+b^4-a^2b-ab^2$

$=a^2(a-b)+b^2(b-a)$

$=(a-b)(a^2-b^2)$

$=(a-b)^2(a+b)\ge0$

Suy ra: $\boxed{a^3+b^4\ge ab(a+b)}$

2.

$a^4+b^4-ab(a^2+b^2)$

$=a^4+b^4-a^3b-ab^3$

$=a^3(a-b)+b^3(b-a)$

$=(a-b)(a^3-b^3)$

$=(a-b)^2(a^2+ab+b^2)\ge0$

Vậy: $\boxed{a^4+b^4\ge ab(a^2+b^2)}$

3.

$a^5+b^5-ab(a^3+b^3)$

$=a^5+b^5-a^4b-ab^4$

$=a^4(a-b)+b^4(b-a)$

$=(a-b)(a^4-b^4)$

$=(a-b)^2(a+b)(a^2+b^2)\ge0$

Vậy: $\boxed{a^5+b^5\ge ab(a^3+b^3)}$

1)

$2x=a+b+c$

$\Rightarrow x-a=\dfrac{b+c-a}{2},\quad x-b=\dfrac{c+a-b}{2},\quad x-c=\dfrac{a+b-c}{2}$

$(x-a)(x-b)+(x-b)(x-c)+(x-c)(x-a)$

$=\dfrac{(b+c-a)(c+a-b)+(c+a-b)(a+b-c)+(a+b-c)(b+c-a)}{4}$

$=\dfrac{2ab+2bc-a^2-b^2-c^2+2bc+2ca-a^2-b^2-c^2+2ca+2ab-a^2-b^2-c^2}{4}$

$=\dfrac{4ab+4bc+4ca-3(a^2+b^2+c^2)}{4}$

$=\dfrac{(a+b+c)^2-2(a^2+b^2+c^2)}{4}$

$=\dfrac{(2x)^2-2(a^2+b^2+c^2)}{4}$

$=ab+bc+ca-x^2$

$\therefore\ (x-a)(x-b)+(x-b)(x-c)+(x-c)(x-a)=ab+ac+bc-x^2.$

2)

$ab+bc+ca=abc,\quad a+b+c=1$

$(a-1)(b-1)(c-1)$$=abc-ab-bc-ca+a+b+c-1$$=abc-ab-bc-ca+1-1$$=abc-(ab+bc+ca)$$=abc-abc$$=0$

$\therefore\ (a-1)(b-1)(c-1)=0.$

Ta có \(\frac{bc}{a^2}+\frac{ab}{c^2}+\frac{ac}{b^2}=\frac{\left(bc\right)^3+\left(ab\right)^3+\left(ac\right)^3}{\left(abc\right)^2}\)

Ta lại có (a+b+c)²=a²+b²+c²

=>a²+b²+c²+2(ab+bc+ac)= a²+b²+c²

=> 2(ab+bc+ac)=0=> ab+bc+ac=0

Ta cần chứng minh bài toán phụ x+y+z=0 thì

x³+y³+z³=3xyz

Ta thấy x+y+z=0=> x+y=-z

=> (x+y)³=-z³ => x³+3xy(x+y)+y³=-z³

=> x³+y³+z³=-3xy(x+y)=-3xy.(-z)=3xyz

Áp dụng vào bài toán ta có 

ab+bc+ac=0 => (ab)³+(bc)³+(ac)³=3(abc)²

=> \(\frac{bc}{a^2}+\frac{ab}{c^2}+\frac{ac}{b^2}=\frac{3\left(abc\right)^2}{\left(abc\right)^2}=3\)

=> đpcm

Ta có: \(a^2,b^2,c^2\le1\Leftrightarrow-1\le a,b,c\le1\)

\(\Rightarrow\left(1+a\right)\left(1+b\right)\left(1+c\right)\ge0\)

\(\Leftrightarrow abc+ab+bc+ca+a+b+c+1\ge0\left(1\right)\)

Ta lại có: \(\frac{\left(a+b+c+1\right)^2}{2}\ge0\)

\(\Leftrightarrow\frac{a^2+b^2+c^2+1+2\left(ab+bc+ca+a+b+c\right)}{2}\ge0\)

\(\Leftrightarrow\frac{1+1+2\left(ab+bc+ca+a+b+c\right)}{2}\ge0\)

\(\Leftrightarrow ab+bc+ca+a+b+c+1\ge0\left(2\right)\)

Lấy (1) + (2) vế theo vế ta được

\(abc+2\left(ab+bc+ca+a+b+c+1\right)\ge0\)

Dấu = xảy ra khi \(\hept{\begin{cases}a=b=0\\c=-1\end{cases}}\) và các hoán vị của nó

2(1+a+b+c+ab+bc+ac)
=2(a^2+b^2+c^2+ab+bc+ac)
=(a^2+b^2+c^2+2ab+2bc+2ac)+2(a+b+c) +1
=(a+b+c)^2+2(a+b+c)+1
=(a+b+c+1)^2 >= 0

đúng thì cho 1 tíck nhé 

Ai giúp mik vs, mik cam on !!!! :)