17．解方程组
（1）$\left\{\begin{array}{l}{x-y=3①}\\{3x-8y=14②}\end{array}\right.$；
（2）$\left\{\begin{array}{l}{x+y=8①}\\{y+z=6②}\\{z+x=4③}\end{array}\right.$．

16．解方程（组）：
（1）$\frac{3x-1}{2}$-$\frac{2x+1}{6}$=-1  
（2）$\left\{\begin{array}{l}{3x+y=7}\\{2x-y=3}\end{array}\right.$
（3）$\frac{x-1}{0.3}$-$\frac{x+2}{0.5}$=1.2
（4）$\left\{\begin{array}{l}{\frac{x}{2}+\frac{y}{3}=2}\\{2x+3y=28}\end{array}\right.$．

15．已知关于x，y的方程x+2y=2，2x+y=7，x-y=2k-1有公共解，求k的值．

14．若关于x和y的二元一次方程组$\left\{\begin{array}{l}{3x+y=2k}\\{2y-x=3}\end{array}\right.$的解满足x＜1且y＞1，求k的整数解．

13．解方程组或不等式组
（1）$\left\{\begin{array}{l}{2x+y=3}\\{3x-5y=11}\end{array}\right.$
（2）$\left\{\begin{array}{l}{3x-1＜2}\\{2x+6＞0}\end{array}\right.$．

12．不等式$\left\{\begin{array}{l}{-3x＞6}\\{2x+8＞0}\end{array}\right.$的解集是-4＜x＜-2．

11．在$\left\{\begin{array}{l}{x=\frac{1}{2}}\\{y=1}\end{array}\right.$，$\left\{\begin{array}{l}{x=5}\\{y=\frac{1}{2}}\end{array}\right.$，$\left\{\begin{array}{l}{x=2}\\{y=-1}\end{array}\right.$，$\left\{\begin{array}{l}{x=1}\\{y=-\frac{1}{3}}\end{array}\right.$中，$\left\{\begin{array}{l}{x=2}\\{y=-1}\end{array}\right.$是二元一次方程组$\left\{\begin{array}{l}{2x+3y=1}\\{x-2y=4}\end{array}\right.$的解．

10．已知?ABCD中，∠A比∠B小20°，那么∠C=80°．

9．解分式方程：
（1）$\frac{1}{x-5}$-$\frac{1}{x-6}$=$\frac{1}{x-8}$-$\frac{1}{x-9}$．
（2）$\frac{x-4}{x-5}$+$\frac{x-8}{x-9}$=$\frac{x-7}{x-8}$+$\frac{x-5}{x-6}$．

8．解下列方程组．
（1）$\left\{\begin{array}{l}{x+y=7}\\{2x-y=8}\end{array}\right.$
（2）$\left\{\begin{array}{l}{x+y=42}\\{0.8x+1.1y=42}\end{array}\right.$．