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【325192】河北省2024八年级数学上册 第13章 轴对称 13.3 等腰三角形13.3.1 等腰

时间:2025-01-15 22:09:31 作者: 字数:2906字
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13章 轴对称

13.3 等腰三角形

13.3.1 等腰三角形

1课时 等腰三角形的性质

 <a href="/tags/55/" title="数学" class="c1" target="_blank">数学</a> <a href="/tags/249/" title="对称" class="c1" target="_blank">对称</a> <a href="/tags/387/" title="三角形" class="c1" target="_blank">三角形</a> <a href="/tags/553/" title="河北" class="c1" target="_blank">河北</a> <a href="/tags/849/" title="轴对称" class="c1" target="_blank">轴对称</a> <a href="/tags/893/" title="河北省" class="c1" target="_blank">河北省</a>

1.三角形按边可分为________________________________________

 <a href="/tags/55/" title="数学" class="c1" target="_blank">数学</a> <a href="/tags/249/" title="对称" class="c1" target="_blank">对称</a> <a href="/tags/387/" title="三角形" class="c1" target="_blank">三角形</a> <a href="/tags/553/" title="河北" class="c1" target="_blank">河北</a> <a href="/tags/849/" title="轴对称" class="c1" target="_blank">轴对称</a> <a href="/tags/893/" title="河北省" class="c1" target="_blank">河北省</a>

1. (1) 等腰三角形的两个底角________(简称“______________”)

(2)等腰三角形的顶角________、底边上的________、底边上的________相互重合(简称“三线合一”)

(3)等腰三角形是轴对称图形,它的对称轴是______________________________所在直线.

2[2024东莞期中]如图,在等腰三角形ABC中,ABAC,∠B25°AD是△ABC的中线,则∠BAD的度数是(  )

 <a href="/tags/55/" title="数学" class="c1" target="_blank">数学</a> <a href="/tags/249/" title="对称" class="c1" target="_blank">对称</a> <a href="/tags/387/" title="三角形" class="c1" target="_blank">三角形</a> <a href="/tags/553/" title="河北" class="c1" target="_blank">河北</a> <a href="/tags/849/" title="轴对称" class="c1" target="_blank">轴对称</a> <a href="/tags/893/" title="河北省" class="c1" target="_blank">河北省</a>

A72° B65° C50° D36°

3[2023西安期末]如图,在△ABC中,ABACAB的垂直平分线DE分别交ACAB于点DE.

(1)若∠A50°,求∠CBD的度数;

(2)AB7BC5,求△CBD的周长.

 <a href="/tags/55/" title="数学" class="c1" target="_blank">数学</a> <a href="/tags/249/" title="对称" class="c1" target="_blank">对称</a> <a href="/tags/387/" title="三角形" class="c1" target="_blank">三角形</a> <a href="/tags/553/" title="河北" class="c1" target="_blank">河北</a> <a href="/tags/849/" title="轴对称" class="c1" target="_blank">轴对称</a> <a href="/tags/893/" title="河北省" class="c1" target="_blank">河北省</a>





 <a href="/tags/55/" title="数学" class="c1" target="_blank">数学</a> <a href="/tags/249/" title="对称" class="c1" target="_blank">对称</a> <a href="/tags/387/" title="三角形" class="c1" target="_blank">三角形</a> <a href="/tags/553/" title="河北" class="c1" target="_blank">河北</a> <a href="/tags/849/" title="轴对称" class="c1" target="_blank">轴对称</a> <a href="/tags/893/" title="河北省" class="c1" target="_blank">河北省</a>

 <a href="/tags/55/" title="数学" class="c1" target="_blank">数学</a> <a href="/tags/249/" title="对称" class="c1" target="_blank">对称</a> <a href="/tags/387/" title="三角形" class="c1" target="_blank">三角形</a> <a href="/tags/553/" title="河北" class="c1" target="_blank">河北</a> <a href="/tags/849/" title="轴对称" class="c1" target="_blank">轴对称</a> <a href="/tags/893/" title="河北省" class="c1" target="_blank">河北省</a>

知识点1 等边对等角

 <a href="/tags/55/" title="数学" class="c1" target="_blank">数学</a> <a href="/tags/249/" title="对称" class="c1" target="_blank">对称</a> <a href="/tags/387/" title="三角形" class="c1" target="_blank">三角形</a> <a href="/tags/553/" title="河北" class="c1" target="_blank">河北</a> <a href="/tags/849/" title="轴对称" class="c1" target="_blank">轴对称</a> <a href="/tags/893/" title="河北省" class="c1" target="_blank">河北省</a> [2023长沙期末]如图,在△ABC中,ABAC,∠A100°,则∠B的度数为(  )

 <a href="/tags/55/" title="数学" class="c1" target="_blank">数学</a> <a href="/tags/249/" title="对称" class="c1" target="_blank">对称</a> <a href="/tags/387/" title="三角形" class="c1" target="_blank">三角形</a> <a href="/tags/553/" title="河北" class="c1" target="_blank">河北</a> <a href="/tags/849/" title="轴对称" class="c1" target="_blank">轴对称</a> <a href="/tags/893/" title="河北省" class="c1" target="_blank">河北省</a>

A40° B50° C60° D80°


               


变式1如图,在△ABC中,ABAC,∠A42°,分别以点A和点B为圆心,大于AB的长为半径作圆弧,两弧相交于点M和点N,作直线MN,交AC于点D,连接BD,则∠DBC的度数为(  )

 <a href="/tags/55/" title="数学" class="c1" target="_blank">数学</a> <a href="/tags/249/" title="对称" class="c1" target="_blank">对称</a> <a href="/tags/387/" title="三角形" class="c1" target="_blank">三角形</a> <a href="/tags/553/" title="河北" class="c1" target="_blank">河北</a> <a href="/tags/849/" title="轴对称" class="c1" target="_blank">轴对称</a> <a href="/tags/893/" title="河北省" class="c1" target="_blank">河北省</a>

A23° B25° C27° D29°

                                                 


               


知识点2 三线合一

 <a href="/tags/55/" title="数学" class="c1" target="_blank">数学</a> <a href="/tags/249/" title="对称" class="c1" target="_blank">对称</a> <a href="/tags/387/" title="三角形" class="c1" target="_blank">三角形</a> <a href="/tags/553/" title="河北" class="c1" target="_blank">河北</a> <a href="/tags/849/" title="轴对称" class="c1" target="_blank">轴对称</a> <a href="/tags/893/" title="河北省" class="c1" target="_blank">河北省</a> [2024来宾期中]如图,在△ABC中,ABACADDBDEAB于点E,若BC3,且△BDC的周长为7,则AE的长为________

 <a href="/tags/55/" title="数学" class="c1" target="_blank">数学</a> <a href="/tags/249/" title="对称" class="c1" target="_blank">对称</a> <a href="/tags/387/" title="三角形" class="c1" target="_blank">三角形</a> <a href="/tags/553/" title="河北" class="c1" target="_blank">河北</a> <a href="/tags/849/" title="轴对称" class="c1" target="_blank">轴对称</a> <a href="/tags/893/" title="河北省" class="c1" target="_blank">河北省</a>


               


变式2[2024南京期中]如图,在△ABC中,ABACAE是中线,BF是角平分线,∠C70°.求∠BAE和∠1的度数.

 <a href="/tags/55/" title="数学" class="c1" target="_blank">数学</a> <a href="/tags/249/" title="对称" class="c1" target="_blank">对称</a> <a href="/tags/387/" title="三角形" class="c1" target="_blank">三角形</a> <a href="/tags/553/" title="河北" class="c1" target="_blank">河北</a> <a href="/tags/849/" title="轴对称" class="c1" target="_blank">轴对称</a> <a href="/tags/893/" title="河北省" class="c1" target="_blank">河北省</a>






13章 轴对称

13.3 等腰三角形

13.3.1 等腰三角形

1课时 等腰三角形的性质

 <a href="/tags/55/" title="数学" class="c1" target="_blank">数学</a> <a href="/tags/249/" title="对称" class="c1" target="_blank">对称</a> <a href="/tags/387/" title="三角形" class="c1" target="_blank">三角形</a> <a href="/tags/553/" title="河北" class="c1" target="_blank">河北</a> <a href="/tags/849/" title="轴对称" class="c1" target="_blank">轴对称</a> <a href="/tags/893/" title="河北省" class="c1" target="_blank">河北省</a>

1.等腰三角形;三边都不相等的三角形

 <a href="/tags/55/" title="数学" class="c1" target="_blank">数学</a> <a href="/tags/249/" title="对称" class="c1" target="_blank">对称</a> <a href="/tags/387/" title="三角形" class="c1" target="_blank">三角形</a> <a href="/tags/553/" title="河北" class="c1" target="_blank">河北</a> <a href="/tags/849/" title="轴对称" class="c1" target="_blank">轴对称</a> <a href="/tags/893/" title="河北省" class="c1" target="_blank">河北省</a>

1(1) 相等;等边对等角 (2) 平分线;中线;高

(3) 底边上的中线(顶角平分线、底边上的高)

2B

3.解:(1)∵ABAC,∠A50°

∴∠ABC=∠C×(180°50°)65°.

又∵DE垂直平分AB,∴DADB,∴∠ABD=∠A50°,∴∠CBD=∠ABC-∠ABD15°.

(2)(1)可知DADB

DBDCDADCAC.

又∵ACAB7BC5,∴△CBD的周长为12.

 <a href="/tags/55/" title="数学" class="c1" target="_blank">数学</a> <a href="/tags/249/" title="对称" class="c1" target="_blank">对称</a> <a href="/tags/387/" title="三角形" class="c1" target="_blank">三角形</a> <a href="/tags/553/" title="河北" class="c1" target="_blank">河北</a> <a href="/tags/849/" title="轴对称" class="c1" target="_blank">轴对称</a> <a href="/tags/893/" title="河北省" class="c1" target="_blank">河北省</a>

1 A 变式1.C 例2 2

变式2.解:∵ABAC,∠C70°,∴∠ABC=∠C70°.

ABACAE是中线,∴AEBC,即∠AEB90°

∴∠BAE90°70°20°.

∵∠ABC70°BF是∠ABC的平分线,∴∠CBF35°

∴∠190°35°125°.