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20. (6分)解下列方程组:
(1)$\left\{\begin{array}{l} 2x+3y=7,\\ x=-2y+3;\end{array}\right. $
解：
(2)$\left\{\begin{array}{l} 2s+3t=-1,\\ 4s-9t=8.\end{array}\right. $
解：
(1)$\left\{\begin{array}{l} 2x+3y=7,\\ x=-2y+3;\end{array}\right. $
解：
$\left\{\begin{array}{l}x = 5\\y = - 1\end{array}\right.$
(2)$\left\{\begin{array}{l} 2s+3t=-1,\\ 4s-9t=8.\end{array}\right. $
解：
$\left\{\begin{array}{l}s=\frac{1}{2}\\t = -\frac{2}{3}\end{array}\right.$
答案
(1)$\left\{\begin{array}{l}x = 5\\y = - 1\end{array}\right.$；(2)$\left\{\begin{array}{l}s=\frac{1}{2}\\t = -\frac{2}{3}\end{array}\right.$
21. (6分)解不等式组$\left\{\begin{array}{l} x+3＞2(x-1),\\ \frac {x-1}{3}＞1,\end{array}\right. $并把解集在数轴上表示出来.
答案
$4\lt x\lt 5$，数轴表示：在数轴上$4$和$5$处画空心圆圈，然后连接这两个空心圆圈之间的线段。
22. (6分)如图,$AB// CD,∠1+∠2=180^{\circ }$,试判断$∠EFM$与$∠NMF$的大小关系,并证明你的结论.
$∠EFM$与$∠NMF$的大小关系是
$∠EFM$与$∠NMF$的大小关系是
$\angle EFM = \angle NMF$
。答案
$\angle EFM = \angle NMF$。
