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数学习题课上李老师出了这样一道题:“如图1,在
中,AD平分
,
.求证:
.
![](https://img.xkw.com/dksih/QBM/2021/1/2/2627320574066688/2635267560464384/STEM/8012fe1d-a928-4dc4-84c2-37642675a713.png)
李老师给出了如下简要分析:“要证
就是要证线段的和差问题,所以有两个方法,方法一:‘截长法’如图2,在AC上截取
,连接DE,只要证
__________即可,这就将证明线段和差问题__________为证明线段相等问题,只要证出
__________
__________,得出
及
_________,再证出
__________
___________,进而得出
,则结论成立.此种证法的基础是‘已知AD平分
,将
沿直线AD对折,使点B落在AC边上的点E处’成为可能.
![](https://img.xkw.com/dksih/QBM/2021/1/2/2627320574066688/2635267560464384/STEM/705dc8bf-dbff-48b5-be72-911ce8484ebe.png)
方法二:“补短法”如图3,延长AB至点F,使
.只要证
即可.此时先证
__________
,再证出
_________
_________,则结论成立.”
![](https://img.xkw.com/dksih/QBM/2021/1/2/2627320574066688/2635267560464384/STEM/e54e7890-8b28-488b-b207-aa1e4b2ce954.png)
“截长补短法”是我们今后证明线段或角的“和差倍分”问题常用的方法.
数学习题课上李老师出了这样一道题:“如图1,在
![](https://staticzujuan.xkw.com/quesimg/Upload/formula/15c0dbe3c080c4c4636c64803e5c1f76.png)
![](https://staticzujuan.xkw.com/quesimg/Upload/formula/7cbce11aa19b8bd2bf6ee5a834e005de.png)
![](https://staticzujuan.xkw.com/quesimg/Upload/formula/88dc4d22c2c289f6d4494b9c2be84407.png)
![](https://staticzujuan.xkw.com/quesimg/Upload/formula/729f26114f36d79bc39470dc5416613b.png)
![](https://img.xkw.com/dksih/QBM/2021/1/2/2627320574066688/2635267560464384/STEM/8012fe1d-a928-4dc4-84c2-37642675a713.png)
李老师给出了如下简要分析:“要证
![](https://staticzujuan.xkw.com/quesimg/Upload/formula/729f26114f36d79bc39470dc5416613b.png)
![](https://staticzujuan.xkw.com/quesimg/Upload/formula/be5fc6ead6416492c231c320a5486f86.png)
![](https://staticzujuan.xkw.com/quesimg/Upload/formula/c978d92edf0c4c1ef8620c17df75d35e.png)
![](https://staticzujuan.xkw.com/quesimg/Upload/formula/ce4cba95fc7d4853a243f8e3fb20ce70.png)
![](https://staticzujuan.xkw.com/quesimg/Upload/formula/dc7861a4ccb7c606518056c58565e03b.png)
![](https://staticzujuan.xkw.com/quesimg/Upload/formula/898611813e24a744cd32df978e1e5c40.png)
![](https://staticzujuan.xkw.com/quesimg/Upload/formula/c978d92edf0c4c1ef8620c17df75d35e.png)
![](https://staticzujuan.xkw.com/quesimg/Upload/formula/665ffcdb7c57534dc184cc840471f2f5.png)
![](https://staticzujuan.xkw.com/quesimg/Upload/formula/9f617fa80f09fc245909492f5335ea5c.png)
![](https://staticzujuan.xkw.com/quesimg/Upload/formula/62f9a3935cd124d021c7b18b0f634915.png)
![](https://staticzujuan.xkw.com/quesimg/Upload/formula/7cbce11aa19b8bd2bf6ee5a834e005de.png)
![](https://staticzujuan.xkw.com/quesimg/Upload/formula/ab2a2834d80ff574e79eae8ca8d4e94f.png)
![](https://img.xkw.com/dksih/QBM/2021/1/2/2627320574066688/2635267560464384/STEM/705dc8bf-dbff-48b5-be72-911ce8484ebe.png)
方法二:“补短法”如图3,延长AB至点F,使
![](https://staticzujuan.xkw.com/quesimg/Upload/formula/7277733875645851ba19f488fc21415d.png)
![](https://staticzujuan.xkw.com/quesimg/Upload/formula/cfa3115a985b37d25d325836409c7766.png)
![](https://staticzujuan.xkw.com/quesimg/Upload/formula/665ffcdb7c57534dc184cc840471f2f5.png)
![](https://staticzujuan.xkw.com/quesimg/Upload/formula/7dc89b88c177060485151747563903d4.png)
![](https://staticzujuan.xkw.com/quesimg/Upload/formula/ce4cba95fc7d4853a243f8e3fb20ce70.png)
![](https://staticzujuan.xkw.com/quesimg/Upload/formula/dc7861a4ccb7c606518056c58565e03b.png)
![](https://img.xkw.com/dksih/QBM/2021/1/2/2627320574066688/2635267560464384/STEM/e54e7890-8b28-488b-b207-aa1e4b2ce954.png)
“截长补短法”是我们今后证明线段或角的“和差倍分”问题常用的方法.
更新时间:2021-01-13 15:28:29
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解答题-证明题
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名校
解题方法
【推荐1】例:截长补短法,是初中几何题中一种添加辅助线的方法,也是把几何题化难为易的一种策略.截长就是在长边上截取一条线段与某一短边相等,补短就是通过延长或旋转等方式使两条短边拼合到一起,从而解决问题.
(1)如图1,△ABC是等边三角形,点D是边BC下方一点,∠BDC=120°,探索线段DA、DB、DC之间的数量关系.
解题思路:将△ABD绕点A逆时针旋转60°得到△ACE,可得AE=AD, CE=BD,∠ABD=∠ACE,∠DAE=60°,根据∠BAC+∠BDC=180°,可知∠ABD+∠ACD=180°,则 ∠ACE+∠ACD=180°,易知△ADE是等边三角形,所以AD=DE,从而解决问题.
根据上述解题思路,三条线段DA、DB、DC之间的等量关系是___________;
(2)如图2,Rt△ABC中,∠BAC=90°,AB=AC.点D是边BC下方一点,∠BDC=90°,探索三条线段DA、DB、DC之间的等量关系,并证明你的结论.
(1)如图1,△ABC是等边三角形,点D是边BC下方一点,∠BDC=120°,探索线段DA、DB、DC之间的数量关系.
解题思路:将△ABD绕点A逆时针旋转60°得到△ACE,可得AE=AD, CE=BD,∠ABD=∠ACE,∠DAE=60°,根据∠BAC+∠BDC=180°,可知∠ABD+∠ACD=180°,则 ∠ACE+∠ACD=180°,易知△ADE是等边三角形,所以AD=DE,从而解决问题.
根据上述解题思路,三条线段DA、DB、DC之间的等量关系是___________;
(2)如图2,Rt△ABC中,∠BAC=90°,AB=AC.点D是边BC下方一点,∠BDC=90°,探索三条线段DA、DB、DC之间的等量关系,并证明你的结论.
![](https://img.xkw.com/dksih/QBM/editorImg/2023/3/7/5de76397-3df9-4b2f-bb65-196b3c81484c.png?resizew=272)
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【推荐2】如图1,在
中,
,D是
的中点,过点B作
,垂足为E,连接
交
于点F.
![](https://img.xkw.com/dksih/QBM/editorImg/2022/7/24/725a06ac-cb73-4a24-a3b1-df2aea7a9072.png?resizew=595)
(1)猜想
与
的数量关系,并说明理由;
(2)P是射线
上的点,过点C作
//
交
的延长线于点G.
①如图2,若点P在
的延长线上,请说明
的理由;
②若
,则
________.
![](https://staticzujuan.xkw.com/quesimg/Upload/formula/15c0dbe3c080c4c4636c64803e5c1f76.png)
![](https://staticzujuan.xkw.com/quesimg/Upload/formula/047dc9795efa99b6fb9fdf9778085dab.png)
![](https://staticzujuan.xkw.com/quesimg/Upload/formula/0dc5c9827dfd0be5a9c85962d6ccbfb1.png)
![](https://staticzujuan.xkw.com/quesimg/Upload/formula/d320f180419175d75eebc618cc458b39.png)
![](https://staticzujuan.xkw.com/quesimg/Upload/formula/03902478df1a55bc99703210bccab910.png)
![](https://staticzujuan.xkw.com/quesimg/Upload/formula/85c4bdfb0db1e31e8459df1d15f9ab55.png)
![](https://img.xkw.com/dksih/QBM/editorImg/2022/7/24/725a06ac-cb73-4a24-a3b1-df2aea7a9072.png?resizew=595)
(1)猜想
![](https://staticzujuan.xkw.com/quesimg/Upload/formula/2d29e7c4d6216690152430b09f9cf0c2.png)
![](https://staticzujuan.xkw.com/quesimg/Upload/formula/5157b42da58d55daad27d98b2fec15ff.png)
(2)P是射线
![](https://staticzujuan.xkw.com/quesimg/Upload/formula/ccaee8f228ff24e7c89879bb5b999cf2.png)
![](https://staticzujuan.xkw.com/quesimg/Upload/formula/abf80148409afb32ced0b4f59f1ba709.png)
![](https://staticzujuan.xkw.com/quesimg/Upload/formula/ccaee8f228ff24e7c89879bb5b999cf2.png)
![](https://staticzujuan.xkw.com/quesimg/Upload/formula/e0629ce42392a7fe9be21d25c39c3e64.png)
①如图2,若点P在
![](https://staticzujuan.xkw.com/quesimg/Upload/formula/ccaee8f228ff24e7c89879bb5b999cf2.png)
![](https://staticzujuan.xkw.com/quesimg/Upload/formula/aa562a732c2af40c63d0152d26f061d9.png)
②若
![](https://staticzujuan.xkw.com/quesimg/Upload/formula/d1c2b9125ec58510d4dc683b7a33f1b9.png)
![](https://staticzujuan.xkw.com/quesimg/Upload/formula/f75158101661ac908d605abfe2048226.png)
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【推荐1】如图,在
中,
,
,边
的垂直平分线
与
交于点
,与
交于点
,连接
.求证:
是等腰三角形.
![](https://staticzujuan.xkw.com/quesimg/Upload/formula/15c0dbe3c080c4c4636c64803e5c1f76.png)
![](https://staticzujuan.xkw.com/quesimg/Upload/formula/12fd0c6cb0e6f9780f4c1c11654c5702.png)
![](https://staticzujuan.xkw.com/quesimg/Upload/formula/c941a2d467928ae2f430dd7a97e3fba3.png)
![](https://staticzujuan.xkw.com/quesimg/Upload/formula/f52a58fbaf4fea03567e88a9f0f6e37e.png)
![](https://staticzujuan.xkw.com/quesimg/Upload/formula/e6e490f703eb6c9bb1278c78ebc2d661.png)
![](https://staticzujuan.xkw.com/quesimg/Upload/formula/f52a58fbaf4fea03567e88a9f0f6e37e.png)
![](https://staticzujuan.xkw.com/quesimg/Upload/formula/2a30f3a8b673cc28bd90c50cf1a35281.png)
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【推荐2】如图,已知点O在等边
的内部,
,
,以
为边作等边
,连接
.
![](https://img.xkw.com/dksih/QBM/editorImg/2023/3/1/006f0817-80b1-4a81-a385-90a85fa85d8c.png?resizew=149)
(1)求证:
;
(2)当
时,试判断
的形状,并说明理由;
![](https://staticzujuan.xkw.com/quesimg/Upload/formula/15c0dbe3c080c4c4636c64803e5c1f76.png)
![](https://staticzujuan.xkw.com/quesimg/Upload/formula/11a1bcaff75f42da99fd3698cce042aa.png)
![](https://staticzujuan.xkw.com/quesimg/Upload/formula/fcbec8352ce3dbfbf3b173045d0ba8f2.png)
![](https://staticzujuan.xkw.com/quesimg/Upload/formula/828628c0876b45381c9a0edeb0fec236.png)
![](https://staticzujuan.xkw.com/quesimg/Upload/formula/3a9a6eeeebf3cff569578d7366b755aa.png)
![](https://staticzujuan.xkw.com/quesimg/Upload/formula/03902478df1a55bc99703210bccab910.png)
![](https://img.xkw.com/dksih/QBM/editorImg/2023/3/1/006f0817-80b1-4a81-a385-90a85fa85d8c.png?resizew=149)
(1)求证:
![](https://staticzujuan.xkw.com/quesimg/Upload/formula/6889321d51141a71e88a1bb1b981fc4b.png)
(2)当
![](https://staticzujuan.xkw.com/quesimg/Upload/formula/7ae35c644141e229ffbd649b69fc9847.png)
![](https://staticzujuan.xkw.com/quesimg/Upload/formula/372629a8666de1e9bac3e7daadcac7b6.png)
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