Đáp án:

Cho a,b,c thỏa mãn:

2ab(2b-a)-2ac(c-2a)-2bc(b-2c)= 7abc

CMR:Tồn tại 1số bằng 2 số kia.

Giải thích các bước giải:

Lời giải:

Theo đề bài ta có:

\(\frac{2ab+1}{2b}=\frac{2bc+1}{c}=\frac{ac+1}{a}\Leftrightarrow a+\frac{1}{2b}=2b+\frac{1}{c}=c+\frac{1}{a}\)

\(\Rightarrow \left\{\begin{matrix} a-2b=\frac{1}{c}-\frac{1}{2b}=\frac{2b-c}{2bc}\\ a-c=\frac{1}{a}-\frac{1}{2b}=\frac{2b-a}{2ab}\\ 2b-c=\frac{1}{a}-\frac{1}{c}=\frac{c-a}{ac}\end{matrix}\right.\)

Nhân theo vế:
\((a-2b)(a-c)(2b-c)=\frac{(2b-c)(2b-a)(c-a)}{4a^2b^2c^2}=\frac{(2b-c)(a-2b)(a-c)}{4a^2b^2c^2}\)

\(\Leftrightarrow (a-2b)(a-c)(2b-c)\left[1-\frac{1}{4a^2b^2c^2}\right]=0\)

$\Rightarrow (a-2b)(a-c)(2b-c)=0$ hoặc $1-\frac{1}{4a^2b^2c^2}=0$

TH1: $(a-2b)(a-c)(2b-c)=0$\(\Rightarrow \left\{\begin{matrix} a=2b\\ a=c\\ 2b=c\end{matrix}\right.\)

+Nếu $a=2b$ thì $\frac{2b-c}{2bc}=a-2b=0\Rightarrow 2b-c=0\Rightarrow 2b=c$

$\Rightarrow a=2b=c$

+ Nếu $a=c, 2b=c$: hoàn toàn tương tự suy ra $a=2b=c$

TH2: $1-\frac{1}{4a^2b^2c^2}=0\Rightarrow 4a^2b^2c^2=1$

Vậy ta có đpcm.

Đặt \(\left(\dfrac{1}{a};\dfrac{1}{2b};\dfrac{1}{c}\right)=\left(x;y;z\right)\Rightarrow x+y+z=0\)

\(M=\dfrac{x^2}{yz}+\dfrac{y^2}{zx}+\dfrac{z^2}{xy}=\dfrac{x^3+y^3+z^3}{xyz}\)

\(=\dfrac{\left(x+y\right)^3-3xy\left(x+y\right)+z^3}{xyz}=\dfrac{-z^3-3xy\left(-z\right)+z^3}{xyz}\)

\(=\dfrac{3xyz}{xyz}=3\)

1.

\(a+b+c=0\)

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

\(\Rightarrow a^2+b^2+c^2+2ab+2bc+2ca=0\)

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

Ta có:

\(\dfrac{\left(a+2b\right)^2+\left(b+2c\right)^2+\left(c+2a\right)^2}{\left(a-2b\right)^2+\left(b-2c\right)^2+\left(c-2a\right)^2}\)

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

\(=\dfrac{5\left(a^2+b^2+c^2\right)+4\left(ab+bc+ca\right)}{5\left(a^2+b^2+c^2\right)-4\left(ab+bc+ca\right)}\)

\(=\dfrac{-10\left(ab+bc+ca\right)+4\left(ab+bc+ca\right)}{-10\left(ab+bc+ca\right)-4\left(ab+bc+ca\right)}\)

\(=\dfrac{-6}{-14}=\dfrac{3}{7}\)

b.

\(a^3+b^3+c^3=3abc\)

\(\Leftrightarrow a^3+b^3+3ab\left(a+b\right)-3ab\left(a+b\right)+c^3-3abc=0\)

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

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

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

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

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

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

\(\Rightarrow\dfrac{ab+2bc+3ca}{3a^2+4b^2+5c^2}=\dfrac{a^2+2a^2+3a^2}{3a^2+4a^2+5a^2}=\dfrac{6}{12}=\dfrac{1}{2}\)