1. 方程组$\begin{cases}x+y=5,\textcircled{1} \\x-y=1 \textcircled{2}\end{cases}$的解是 ( )
A.$\begin{cases}x=1, \\y=4 \end{cases}$
B.$\begin{cases}x=2, \\y=3 \end{cases}$
C.$\begin{cases}x=3, \\y=2 \end{cases}$
D.$\begin{cases}x=4, \\y=1 \end{cases}$

1. C

3. 已知二元一次方程组$\begin{cases}2x-y=5,\textcircled{1} \\x-2y=1,\textcircled{2}\end{cases}$则$x+y$的值为 ( )
A.$2$
B.$6$
C.$4$
D.$-6$

3. C

4. 解二元一次方程组$\begin{cases}13x-6y=25,\textcircled{1} \\27x-4y=19,\textcircled{2}\end{cases}$较简便的方法是先消去$y$,即利用$\textcircled{2}×3-\textcircled{1}×2$,最后化简得$x=$ ______ .

4. $\dfrac{7}{55}$

5. 某地因盛产有机黑胡椒和有机白胡椒而闻名.已知每千克有机黑胡椒比有机白胡椒的售价便宜10元,购买$2\ \mathrm{kg}$有机黑胡椒和$3\ \mathrm{kg}$有机白胡椒需付280元,求每千克有机黑胡椒和每千克有机白胡椒的售价.

5. 解:每千克有机黑胡椒的售价为50元,每
千克有机白胡椒的售价为60元.

6. 解下列二元一次方程组:
(1)$\begin{cases}3x+2y=14, & \textcircled{1} \\x=y+3; & \textcircled{2} \end{cases}$
(2)$\begin{cases}x-y=2, & \textcircled{1} \\2x+y=7; & \textcircled{2} \end{cases}$
(3)$\begin{cases}4x+3y=5, & \textcircled{1} \\2x-y=-5; & \textcircled{2} \end{cases}$
(4)$\begin{cases}2x-7y=5, & \textcircled{1} \\3x-8y=10; & \textcircled{2} \end{cases}$
(5)$\begin{cases}\dfrac{x}{2}-\dfrac{y}{3}=\dfrac{1}{3}, & \textcircled{1} \\3(x-1)=y+1; & \textcircled{2} \end{cases}$
(6)$\begin{cases}3x-2y+20=0, & \textcircled{1} \\2x+15y-3=0. & \textcircled{2} \end{cases}$

6. 解:(1)$\begin{cases} x=4,\\ y=1.\\ \end{cases}$ (2)$\begin{cases} x=3,\\ y=1.\\ \end{cases}$ (3)$\begin{cases} x=-1,\\ y=3.\\ \end{cases}$
(4)$\begin{cases} x=6,\\ y=1.\\ \end{cases}$ (5)$\begin{cases} x=2,\\ y=2.\\ \end{cases}$ (6)$\begin{cases} x=-6,\\ y=1.\\ \end{cases}$