1. 在下列方程组中，是三元一次方程组的是（ ）。

A.$ x+y-3z=1 $
B.$ \{\begin{array}{l l}3x^{2}-yz=2,\\y+2z=4\end{array} $
C.$ \{\begin{array}{l l}abc=-1,\\a+2b+3c=4\end{array} $
D.$ \{\begin{array}{l l}a-2b-c=3,\\a+b-3c=1,\\a-b+c=2\end{array} $

1. D

 2. 用消元法解三元一次方程组 $ \{\begin{array}{l l}x-y-z=3,\\x+2y+z=5,\\3x+y-z=-2,\end{array} $若先消去 z，下列正确的是（ ）.

A.$ \{\begin{array}{l l}4x=1,\\2x-y=3\end{array} $
B.$ \{\begin{array}{l l}5x+2y=6,\\4x+3y=3\end{array} $
C.$ \{\begin{array}{l l}2x+y=8,\\2x-y=3\end{array} $
D.$ \{\begin{array}{l l}2x+y=8,\\4x+3y=3\end{array} $

2. D

 3. 在下列各组数中，是三元一次方程组 $ \{\begin{array}{l l}x+2 y-z=3,\\2 x-y+z=2,\\x+y+z=5\end{array} $的解的是（ ）.

A.$ \{\begin{array}{l l}x=1,\\y=2,\\z=2\end{array} $
B.$ \{\begin{array}{l l}x=-3,\\y=14,\\z=-6\end{array} $
C.$ \{\begin{array}{l l}x=-1,\\y=8,\\z=12\end{array} $
D.$ \{\begin{array}{l l}x=2,\\y=1,\\z=1\end{array} $

3. A

 4. 在下列方程组中，解为 $ \{\begin{array}{l l}x=-1,\\ y=3,\\ z=2\end{array} $的方程组的是（ ）.

A.$ \{\begin{array}{l l}6 x-y+5 z=1,\\ 3 x+8 y-1 1 z=5,\\ x-y+2 z=6\end{array} $
B.$ \{\begin{array}{l l}6 x-y+5 z=1,\\ 3 x+2 y+z=5,\\ 2 x-y+z=-3\end{array} $
C.$ \{\begin{array}{l l}3 x+2 y-z=1,\\ 3 x-2 y+z=5,\\ 2 x-y+z=-3\end{array} $
D.$ \{\begin{array}{l l}3 x+2 y-z=1,\\ 3 x+8 y-1 1 z=5,\\ x-y+z=2\end{array} $

4. B

 5. 解三元一次方程组 $ \{\begin{array}{l l}x+y-z=8,&\textcircled{1}\\x-y+3 z=1 0,&\textcircled{2}\\x=2 z&\textcircled{3}\end{array} $时，把 $ \textcircled{3} $分别代入 $ \textcircled{1}\textcircled{2} $，得到只含 y，z的方程组，它是（ ）.

A.$ \{\begin{array}{l l}y-z=8,\\-y+3 z=1 0\end{array} $
B.$ \{\begin{array}{l l}y+z=8,\\-y+3 z=1 0\end{array} $
C.$ \{\begin{array}{l l}y+z=8,\\-y+5 z=1 0\end{array} $
D.$ \{\begin{array}{l l}y-z=8,\\-y+5 z=1 0\end{array} $

5. C

 6. 解三元一次方程组 $ \{\begin{array}{l l}2 x-y=7,&\textcircled{1}\\x-y+2 z=0,&\textcircled{2}\\3 x+y+2 z=1 6,&\textcircled{3}\end{array} $由 $ \textcircled{3} $ $ \textcircled{2} $ ，得_______；得到的只含 x，y的方程组为_______；解这个二元一次方程组，得_______；原三元一次方程组的解为_______.

6. $x+y=8$; $\begin{cases}2x-y=7, \\x+y=8;\end{cases}$ $\begin{cases}x=5, \\y=3;\end{cases}$ $\begin{cases}x=5, \\y=3, \\z=-1\end{cases}$

 7. 解三元一次方程组 $ \{\begin{array}{l l}2 x-y+5 z=2 0, \textcircled{1}\\x+y-z=-2, \textcircled{2}\\x+2 y+2 z=6, \textcircled{3}\end{array} $由 $ \textcircled{1}+\textcircled{2} $ ，得_______；由 $ \textcircled{1} ×2+\textcircled{3} $ ，得 ___；得到的只含 x，z的方程组为_______；解这个二元一次方程组，得 ___；原三元一次方程组的解为_______.

7. $3x+4z=18;5x+12z=46;$
$\begin{cases}3x+4z=18, \\5x+12z=46;\end{cases}$ $\begin{cases}x=2, \\z=3;\end{cases}$ $\begin{cases}x=2, \\y=-1, \\z=3\end{cases}$