2. 已知$\begin{cases}3x + 3y = z, \\3x + 4y = 2z + 6,\end{cases}$且$z + y = 0$，则$z$的值为【 】

A.9
B.-3
C.12
D.不确定

2. B

3. 三元一次方程组$\begin{cases}5x + 4y + z = 0, \\3x + y - 4z = 11, \\x + y + z = -2,\end{cases}$消去未知数$z$后，得到的二元一次方程组是【 】

A.$\begin{cases}4x + 3y = 2, \\7x + 5y = 3\end{cases}$
B.$\begin{cases}4x + 3y = 2, \\23x + 17y = 10\end{cases}$
C.$\begin{cases}3x + 4y = 2, \\7x + 5y = 3\end{cases}$
D.$\begin{cases}3x + 4y = 2, \\23x + 17y = 11\end{cases}$

3. A

4. 解三元一次方程组$\begin{cases}x + y + 2z = 7, \\2x + 3y - z = 12, \\3x + 2y + z = 13\end{cases}$时，宜先消去未知数 ______ ，得到关于 ______ 的二元一次方程组 ______ ，解这个方程组得 ______ ，则原方程组的解为 ______ .

4. z x、y $\{\begin{array}{l} x+y=5,\\ 5x+7y=31\end{array} $ $\{\begin{array}{l} x=2,\\ y=3\end{array} $ $\{\begin{array}{l} x=2,\\ y=3,\\ z=1\end{array} $

5. 三元一次方程组$\begin{cases}3x - y + z = 4, \\2x + y - z = 11, \\x + y - 2z = 3\end{cases}$的解是 ______ .

5. $\{\begin{array}{l} x=3,\\ y=10,\\ z=5\end{array} $

6. 已知$\frac{x + y}{2} = \frac{y + z}{3} = \frac{x + z}{4} = -2$，求$x + 2y + z$的值.

6. -10

7. 解下列三元一次方程组：
(1) $\begin{cases}x - 4y + z = -3, \\2x + y - z = 18, \\x - y - z = 7;\end{cases}$

(2) $\begin{cases}x + 2y - z = 0, \\2x - 3z = 1, \\3x - y + 2z = 4.\end{cases}$

7. (1) $\{\begin{array}{l} x=7,\\ y=2,\\ z=-2\end{array} $ (2) $\{\begin{array}{l} x=1,\\ y=-\frac {1}{3},\\ z=\frac {1}{3}\end{array} $