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七年级数学solution一般
题目
已知,在ABC\triangle ABC中,BAC=30\angle BAC=30^{\circ},点DD在射线BCBC上,连接ADAD,CAD=α\angle CAD=\alpha,点DD关于直线ACAC的对称点为EE,点EE关于直线ABAB的对称点为FF,直线EFEF分别交直线ACAC,ABAB于点MM,NN,连接AFAF,AEAE,CECE.
(1)(1)如图11,点DD在线段BCBC上.
①根据题意补全图11
AEF=\angle AEF=______(用含有α\alpha的代数式表示),
AMF=______\angle AMF=\_\_\_\_\_\_^{\circ}
③用等式表示线段MAMA,MEME,MFMF之间的数量关系,并证明.
(2)(2)DD在线段BCBC的延长线上,且CAD<60\angle CAD \lt 60^{\circ},直接用等式表示线段MAMA,MEME,MFMF之间的数量关系,不证明.
知识点:全等三角形的性质、全等三角形的判定、勾股定理、菱形的性质、平移的性质、旋转的性质、相似三角形的性质I、相似三角形的判定I、相似三角形的判定与性质章节:未标注

答案与解析

答案

(1)①如图所示:

\becauseDD关于直线ACAC的对称点为EE
ADC\therefore \triangle ADCAEC\triangle AEC
CAE=CAD=α\therefore \angle CAE=\angle CAD=\alpha
BAC=30\because \angle BAC=30^{\circ}
EAN=30+α\therefore \angle EAN=30^{\circ}+\alpha
\becauseEE关于直线ABAB的对称点为FF
AB\therefore AB垂直平分EFEF
AF=AE\therefore AF=AEFAN=EAN=30+α\angle FAN=\angle EAN=30^{\circ}+\alpha
F=AEF=180°2(30°+α)2=60°α\therefore \angle F=\angle AEF=\frac{{180°-2({30°+α})}}{2}=60°-α
AMF=60α+α=60\therefore \angle AMF=60^{\circ}-\alpha +\alpha =60^{\circ}
故答案为:60α60^{\circ}-\alpha6060
MF=MA+MEMF=MA+ME,理由如下:如图,在FEFE上截取GF=MEGF=ME,连接AGAG

\becauseDD关于直线ACAC的对称点为EE
ADC\therefore \triangle ADCAEC\triangle AEC
CAE=CAD=α\therefore \angle CAE=\angle CAD=\alpha
BAC=30\because \angle BAC=30^{\circ}
EAN=30+α\therefore \angle EAN=30^{\circ}+\alpha
\becauseEE关于直线ABAB的对称点为FF
AB\therefore AB垂直平分EFEF
AF=AE\therefore AF=AEFAN=EAN=30+α\angle FAN=\angle EAN=30^{\circ}+\alpha
F=AEF=180°2(30°+α)2=60°α\therefore \angle F=\angle AEF=\frac{{180°-2({30°+α})}}{2}=60°-α.
AMG=60α+α=60\therefore \angle AMG=60^{\circ}-\alpha +\alpha =60^{\circ}
AF=AE\because AF=AEF=AEF\angle F=\angle AEFGF=MEGF=ME
AFG\therefore \triangle AFGAEM(SAS)\triangle AEM\left(SAS\right)
AG=AM\therefore AG=AM
AMG=60\because \angle AMG=60^{\circ}
AGM\therefore \triangle AGM为等边三角形,
MA=MG\therefore MA=MG
MF=MG+GF=MA+ME\therefore MF=MG+GF=MA+ME
(2)MF=MAME(2)MF=MA-ME,理由如下:
如图22,延长MFMFGG,使FG=EMFG=EM,连接AGAG

\becauseDD关于直线ACAC的对称点为EE
CAE=CAD=α\therefore \angle CAE=\angle CAD=\alpha
BAC=30\because \angle BAC=30^{\circ}
EAN=30α\therefore \angle EAN=30^{\circ}-\alpha
\becauseEE关于直线ABAB的对称点为FF
AB\therefore AB垂直平分EFEF
AF=AE\therefore AF=AEFAN=EAN=30α\angle FAN=\angle EAN=30^{\circ}-\alpha
AFE=AEF=180°2(30°α)2=60+α\therefore \angle AFE=\angle AEF=\frac{180°-2(30°-α)}{2}=60^{\circ}+\alpha
AMF=AEFCAE=60+αα=60\therefore \angle AMF=\angle AEF-\angle CAE=60^{\circ}+\alpha -\alpha =60^{\circ}AEM=AFG\angle AEM=\angle AFG
AF=AE\because AF=AEGF=MEGF=ME
AFG\therefore \triangle AFGAEM(SAS)\triangle AEM\left(SAS\right)
AG=AM\therefore AG=AM
AMG=60\because \angle AMG=60^{\circ}
AGM\therefore \triangle AGM为等边三角形,
MA=MG\therefore MA=MG
MF=MGGF=MAME\therefore MF=MG-GF=MA-ME.

(1)(1)①依照题意画出图形即可;
②由轴对称的性质可求AF=AEAF=AEFAN=EAN=30+α\angle FAN=\angle EAN=30^{\circ}+\alpha,由等腰三角形的性质可求解;
③由“SASSAS”可证AFG\triangle AFGAEM\triangle AEM,可得MA=MGMA=MG,由线段的数量关系可求解;
(2)(2)由“SASSAS”可证AFG\triangle AFGAEM\triangle AEM,可得MA=MGMA=MG,由线段的数量关系可求解.

【点评】

本题是几何变换综合题,考查了轴对称的性质,等边三角形的判定和性质,全等三角形的判定和性质等知识,灵活运用这些性质解决问题是解题的关键.

解析

(1)①如图所示:

\becauseDD关于直线ACAC的对称点为EE
ADC\therefore \triangle ADCAEC\triangle AEC
CAE=CAD=α\therefore \angle CAE=\angle CAD=\alpha
BAC=30\because \angle BAC=30^{\circ}
EAN=30+α\therefore \angle EAN=30^{\circ}+\alpha
\becauseEE关于直线ABAB的对称点为FF
AB\therefore AB垂直平分EFEF
AF=AE\therefore AF=AEFAN=EAN=30+α\angle FAN=\angle EAN=30^{\circ}+\alpha
F=AEF=180°2(30°+α)2=60°α\therefore \angle F=\angle AEF=\frac{{180°-2({30°+α})}}{2}=60°-α
AMF=60α+α=60\therefore \angle AMF=60^{\circ}-\alpha +\alpha =60^{\circ}
故答案为:60α60^{\circ}-\alpha6060
MF=MA+MEMF=MA+ME,理由如下:如图,在FEFE上截取GF=MEGF=ME,连接AGAG

\becauseDD关于直线ACAC的对称点为EE
ADC\therefore \triangle ADCAEC\triangle AEC
CAE=CAD=α\therefore \angle CAE=\angle CAD=\alpha
BAC=30\because \angle BAC=30^{\circ}
EAN=30+α\therefore \angle EAN=30^{\circ}+\alpha
\becauseEE关于直线ABAB的对称点为FF
AB\therefore AB垂直平分EFEF
AF=AE\therefore AF=AEFAN=EAN=30+α\angle FAN=\angle EAN=30^{\circ}+\alpha
F=AEF=180°2(30°+α)2=60°α\therefore \angle F=\angle AEF=\frac{{180°-2({30°+α})}}{2}=60°-α.
AMG=60α+α=60\therefore \angle AMG=60^{\circ}-\alpha +\alpha =60^{\circ}
AF=AE\because AF=AEF=AEF\angle F=\angle AEFGF=MEGF=ME
AFG\therefore \triangle AFGAEM(SAS)\triangle AEM\left(SAS\right)
AG=AM\therefore AG=AM
AMG=60\because \angle AMG=60^{\circ}
AGM\therefore \triangle AGM为等边三角形,
MA=MG\therefore MA=MG
MF=MG+GF=MA+ME\therefore MF=MG+GF=MA+ME
(2)MF=MAME(2)MF=MA-ME,理由如下:
如图22,延长MFMFGG,使FG=EMFG=EM,连接AGAG

\becauseDD关于直线ACAC的对称点为EE
CAE=CAD=α\therefore \angle CAE=\angle CAD=\alpha
BAC=30\because \angle BAC=30^{\circ}
EAN=30α\therefore \angle EAN=30^{\circ}-\alpha
\becauseEE关于直线ABAB的对称点为FF
AB\therefore AB垂直平分EFEF
AF=AE\therefore AF=AEFAN=EAN=30α\angle FAN=\angle EAN=30^{\circ}-\alpha
AFE=AEF=180°2(30°α)2=60+α\therefore \angle AFE=\angle AEF=\frac{180°-2(30°-α)}{2}=60^{\circ}+\alpha
AMF=AEFCAE=60+αα=60\therefore \angle AMF=\angle AEF-\angle CAE=60^{\circ}+\alpha -\alpha =60^{\circ}AEM=AFG\angle AEM=\angle AFG
AF=AE\because AF=AEGF=MEGF=ME
AFG\therefore \triangle AFGAEM(SAS)\triangle AEM\left(SAS\right)
AG=AM\therefore AG=AM
AMG=60\because \angle AMG=60^{\circ}
AGM\therefore \triangle AGM为等边三角形,
MA=MG\therefore MA=MG
MF=MGGF=MAME\therefore MF=MG-GF=MA-ME.

(1)(1)①依照题意画出图形即可;
②由轴对称的性质可求AF=AEAF=AEFAN=EAN=30+α\angle FAN=\angle EAN=30^{\circ}+\alpha,由等腰三角形的性质可求解;
③由“SASSAS”可证AFG\triangle AFGAEM\triangle AEM,可得MA=MGMA=MG,由线段的数量关系可求解;
(2)(2)由“SASSAS”可证AFG\triangle AFGAEM\triangle AEM,可得MA=MGMA=MG,由线段的数量关系可求解.

【点评】

本题是几何变换综合题,考查了轴对称的性质,等边三角形的判定和性质,全等三角形的判定和性质等知识,灵活运用这些性质解决问题是解题的关键.

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