20．若不等式组$\left\{{\begin{array}{l}{x＞a}\\{x＜b}\end{array}}\right.$的解集为-1＜x＜1，则（a+1）（b-1）=0．

19．不等式组$\left\{{\begin{array}{l}{x＞a}\\{x＜b}\end{array}}\right.$有解的条件是a＜b，无解的条件是a≥b．

18．若$\left\{{\begin{array}{l}{x=2}\\{y=1}\end{array}}\right.$是方程2x-ay=3的一个解，则a=1；当x=2时，代数式 2x+1与x-7的值互为相反数．

17．（1）解方程组：$\left\{\begin{array}{l}{x+2y=0}\\{3x+4y=6}\end{array}\right.$
 （2）解不等式组：$\left\{\begin{array}{l}{2（x+1）≥x+3}\\{\frac{2x+1}{3}＞x-1}\end{array}\right.$，并写出它的所有整数解．

16．解方程组或不等式组，并把不等式组的解集在数轴上表示出来：
（1）$\left\{\begin{array}{l}{3x+2y=16}\\{x-3y=-2}\end{array}\right.$
（2）$\left\{\begin{array}{l}{5x-1＞2x-4}\\{\frac{1}{2}x≤\frac{x+2}{4}}\end{array}\right.$．

15．已知关于x、y的二元一次方程组$\left\{\begin{array}{l}{2x+y=2a}\\{3x+2y=4-2a}\end{array}\right.$的解互为相反数，求x、y、a的值．

14．解下列方程组：
①$\left\{\begin{array}{l}{3x-2y=6}\\{2x-3y=17}\end{array}\right.$           
②$\left\{\begin{array}{l}{\frac{1}{2}x+\frac{3}{2}y=4}\\{5x-3y=4}\end{array}\right.$．

13．不等式组$\left\{\begin{array}{l}{x-1＜0}\\{x+2＞0}\end{array}\right.$的解集是-2＜x＜1；整数解为-1，0．

12．已知关于x，y的方程组 $\left\{\begin{array}{l}x+2y=3m\\ x-y=9m\end{array}\right.$的解是方程3x+2y=34的一组解，求m的值．

11．解方程组：
（1）$\left\{\begin{array}{l}3x-4y=10\\ 5x+6y=42.\end{array}\right.$
（2）$\left\{\begin{array}{l}x=2y-3\\ 7x+5y=-2\end{array}\right.$
（3）$\left\{\begin{array}{l}{\frac{x}{2}-\frac{y+1}{3}=1}\\{3x+2y=10}\end{array}\right.$                 
（4）$\left\{\begin{array}{l}{6（x+y）=4（2x-y）+16}\\{\frac{2（x-y）}{3}=\frac{x+y}{4}-1}\end{array}\right.$．