7.2 二元一次方程组的解法 代入法（第3课时）
学习目标
1．使学生进一步会用代入消元法解二元一次方程组，理解解二元一次方程组的消元思想,
2．通过代入消元，使学生理解化归思想
重点：熟练地用代入法消元法解二元一次方程组.
难点：理解解方程组的基本思想是“消元”，体会化归思想。
一、新知准备与自学：（学生自学教材![](https://wkrtcs.bdimg.com/rtcs/image?w=3&md5sum=02549618461689b032c5827155bec544&sign=b35f393b72&rtcs_flag=1&rtcs_ver=3.1&l=webapp&bucketNum=42&ipr={"t":"img","w":"3","h":"1","dataType":"png","c":"word/media/image1.png","imgOriW":324,"imgOriH":36})
并完成填空后互评）时间：10分钟
1 把下列方程写成用含![](https://wkrtcs.bdimg.com/rtcs/image?w=13&md5sum=02549618461689b032c5827155bec544&sign=b35f393b72&rtcs_flag=1&rtcs_ver=3.1&l=webapp&bucketNum=42&ipr={"t":"img","w":"13","h":"15","dataType":"wmf","c":"word/media/image2.png","imgOriW":26,"imgOriH":30})
的代数式表示的形式:
(1) ![](https://wkrtcs.bdimg.com/rtcs/image?w=97&md5sum=02549618461689b032c5827155bec544&sign=b35f393b72&rtcs_flag=1&rtcs_ver=3.1&l=webapp&bucketNum=42&ipr={"t":"img","w":"97","h":"21","dataType":"wmf","c":"word/media/image4.png","imgOriW":194,"imgOriH":42})
； (2)
2、 解方程组：![](https://wkrtcs.bdimg.com/rtcs/image?w=130&md5sum=02549618461689b032c5827155bec544&sign=b35f393b72&rtcs_flag=1&rtcs_ver=3.1&l=webapp&bucketNum=42&ipr={"t":"img","w":"130","h":"44","dataType":"wmf","c":"word/media/image6.png","imgOriW":260,"imgOriH":88})
①②
解：由①，用含y的代数式表示x得 ③
将③代入②，得
解得 ： ![](https://wkrtcs.bdimg.com/rtcs/image?w=3&md5sum=02549618461689b032c5827155bec544&sign=b35f393b72&rtcs_flag=1&rtcs_ver=3.1&l=webapp&bucketNum=42&ipr={"t":"img","w":"3","h":"2","dataType":"png","c":"word/media/image1.png","imgOriW":324,"imgOriH":36})
y＝ .
将y＝ 代入③，得
得x＝ .
所以 ![](https://wkrtcs.bdimg.com/rtcs/image?w=35&md5sum=02549618461689b032c5827155bec544&sign=b35f393b72&rtcs_flag=1&rtcs_ver=3.1&l=webapp&bucketNum=42&ipr={"t":"img","w":"35","h":"48","dataType":"wmf","c":"word/media/image7.png","imgOriW":70,"imgOriH":96})
三 合作 探索展示（学生独立思考后小组交流教师巡视根据情况点拨）时间：10-15分钟
1 解方程组
(1)![](https://wkrtcs.bdimg.com/rtcs/image?w=125&md5sum=02549618461689b032c5827155bec544&sign=b35f393b72&rtcs_flag=1&rtcs_ver=3.1&l=webapp&bucketNum=42&ipr={"t":"img","w":"125","h":"48","dataType":"wmf","c":"word/media/image8.png","imgOriW":250,"imgOriH":96})
. （2）
三 、 知识与巩固应用。（学生独立完成后小组交流师生评正）20分钟
1、已知![](https://wkrtcs.bdimg.com/rtcs/image?w=12&md5sum=02549618461689b032c5827155bec544&sign=b35f393b72&rtcs_flag=1&rtcs_ver=3.1&l=webapp&bucketNum=42&ipr={"t":"img","w":"12","h":"19","dataType":"wmf","c":"word/media/image10.png","imgOriW":24,"imgOriH":38})
是二元一次方程组的解，则的值为（ ）
A、 1 B、－1 C、2 D、3
2、已知代数式![](https://wkrtcs.bdimg.com/rtcs/image?w=60&md5sum=02549618461689b032c5827155bec544&sign=b35f393b72&rtcs_flag=1&rtcs_ver=3.1&l=webapp&bucketNum=42&ipr={"t":"img","w":"60","h":"24","dataType":"wmf","c":"word/media/image14.png","imgOriW":120,"imgOriH":48})
与是同类项，那么的值分别是（ ）
A．![](https://wkrtcs.bdimg.com/rtcs/image?w=53&md5sum=02549618461689b032c5827155bec544&sign=b35f393b72&rtcs_flag=1&rtcs_ver=3.1&l=webapp&bucketNum=42&ipr={"t":"img","w":"53","h":"48","dataType":"wmf","c":"word/media/image17.png","imgOriW":106,"imgOriH":96})
B． C． D．
3、 若方程组![](https://wkrtcs.bdimg.com/rtcs/image?w=84&md5sum=02549618461689b032c5827155bec544&sign=b35f393b72&rtcs_flag=1&rtcs_ver=3.1&l=webapp&bucketNum=42&ipr={"t":"img","w":"84","h":"48","dataType":"wmf","c":"word/media/image21.png","imgOriW":168,"imgOriH":96})
与方程组的解相同，则__________.
4、若![](https://wkrtcs.bdimg.com/rtcs/image?w=56&md5sum=02549618461689b032c5827155bec544&sign=b35f393b72&rtcs_flag=1&rtcs_ver=3.1&l=webapp&bucketNum=42&ipr={"t":"img","w":"56","h":"48","dataType":"wmf","c":"word/media/image24.png","imgOriW":112,"imgOriH":96})
是关于、的方程的一个解，且，则＝ 。
5、 解方程组
(1)![](https://wkrtcs.bdimg.com/rtcs/image?w=93&md5sum=02549618461689b032c5827155bec544&sign=b35f393b72&rtcs_flag=1&rtcs_ver=3.1&l=webapp&bucketNum=42&ipr={"t":"img","w":"93","h":"48","dataType":"wmf","c":"word/media/image30.png"})
（2）
（3）![](https://wkrtcs.bdimg.com/rtcs/image?w=121&md5sum=02549618461689b032c5827155bec544&sign=b35f393b72&rtcs_flag=1&rtcs_ver=3.1&l=webapp&bucketNum=42&ipr={"t":"img","w":"121","h":"48","dataType":"wmf","c":"word/media/image32.png"})
(4)
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