a)\(x^2+2y^2+2xy-2y+1=0\)

\(\Leftrightarrow x^2+2xy+y^2+y^2-2y+1=0\)

\(\Leftrightarrow\left(x+y\right)^2+\left(y-1\right)^2=0\)

\(\Leftrightarrow\hept{\begin{cases}y-1=0\\x+y=0\end{cases}}\Leftrightarrow\hept{\begin{cases}y=1\\x=-y=-1\end{cases}}\)

Vậy x=-1 y=1

a)  \(x^2+2y^2+2xy-2y+1=0\)

\(\Leftrightarrow\left(x^2+2xy+y^2\right)+\left(y^2-2y+1\right)=0\)

\(\Leftrightarrow\left(x+y\right)^2+\left(y-1\right)^2=0\)

\(\Rightarrow\orbr{\begin{cases}\left(x+y\right)^2=0\\\left(y-1\right)^2=0\end{cases}}\Leftrightarrow\orbr{\begin{cases}x+y=0\\y-1=0\end{cases}\Leftrightarrow\orbr{\begin{cases}x=-y\\y=1\end{cases}\Rightarrow}x=-1;y=1}\)

b) \(5x^2+3y^2+z^2-4x+6xy+4z+6=0\)

\(\Leftrightarrow\left(2x^2-4x+2\right)+\left(3x^2+6xy+3y^2\right)+\left(z^2+4z+4\right)=0\)

\(\Leftrightarrow2.\left(x-1\right)^2+3.\left(x+y\right)^2+\left(z+2\right)^2=0\)

\(\Rightarrow\)  \(\left(x-1\right)^2=0\Rightarrow x-1=0\Rightarrow x=1\)

           \(\left(x+y\right)^2=0\Rightarrow x+y=0\Rightarrow y=-x=-1\) 

            \(\left(z+2\right)^2=0\Rightarrow z+2=0\Rightarrow z=-2\)

a) Áp dụng tính chất dãy tỉ số bằng nhau ta có : \(\frac{x}{20}=\frac{y}{9}=\frac{z}{6}=\frac{x-2y+4z}{20-2.9+4.6}=\frac{13}{26}=\frac{1}{2}\)

* \(\frac{x}{20}=\frac{1}{2}\Rightarrow x=\frac{1}{2}.20=10\)

*\(\frac{y}{9}=\frac{1}{2}\Rightarrow y=\frac{1}{2}.9=\frac{9}{2}\)

*\(\frac{z}{6}=\frac{1}{2}\Rightarrow z=\frac{1}{2}.6=3\)

b)c) đề bn viết ko rõ

\(\frac{x-1}{2}=\frac{y-2}{3}=\frac{z-3}{4}\)

Áp dụng tính chất dãy tỉ số bằng nhau ta có: 

\(\frac{x-1}{2}=\frac{y-2}{3}=\frac{z-3}{4}=\frac{x-1-2\left(y-2\right)+3\left(z-3\right)}{2-2.3+3.4}=\frac{x-2y+3z-6}{8}=1\)

\(\Leftrightarrow\hept{\begin{cases}x-1=2\\y-2=3\\z-3=4\end{cases}}\Leftrightarrow\hept{\begin{cases}x=3\\y=5\\z=7\end{cases}}\)

Từ \(\frac{x}{2}=\frac{y}{3}=\frac{z}{5}\Rightarrow\frac{x}{2}=\frac{2y}{6}=\frac{z}{5}\)

Theo t/c dãy tỉ số=nhau:

\(\frac{x}{2}=\frac{2y}{6}=\frac{z}{5}=\frac{x-2y+z}{2-6+5}=\frac{10}{1}=10\)

+)x/2=10=>x=20

+)2y/6=10=>2y=60=>y=30

+)z/5=10=>z=50

Vậy................

\(\frac{x}{2}=\frac{y}{3}=\frac{z}{5}=\frac{2y}{6}=\frac{x-2y+z}{2-6+5}=10\)

\(\Rightarrow x=20\)       \(y=30\)            \(z=50\)