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八年级数学选择题一般
题目
如图,在ABC\triangle ABC中,ABC=60\angle ABC=60^{\circ},ADAD平分BAC\angle BACBCBC于点DD,CECE平分ACB\angle ACBABAB于点EE,ADADCECE交于点FF.则下列说法正确的个数为( )
AFC=120\angle AFC=120^{\circ}
SABD=SADCS_{\triangle ABD}=S_{\triangle ADC}
③若CEABCE\bot AB,则AB=2AEAB=2AE
CD+AE=ACCD+AE=AC
SAEFS_{\triangle AEF}:SFDC=AF:FCS_{\triangle FDC}=AF:FC.
A.
22
B.
33
C.
44
D.
55
知识点:线段垂直平分线的性质、全等三角形的性质、全等三角形的判定、直角三角形全等的判定、等腰三角形的性质、等腰三角形的判定定理、直角三角形的性质、三角形的面积章节:第13章 三角形 / 13.1 三角形的概念

答案与解析

答案

C

解析

BAC=2α\angle BAC=2\alphaACB=2β\angle ACB=2\beta
AD\because AD平分BAC\angle BACBCBC于点DDCECE平分ACB\angle ACBABAB于点EEABC=60\angle ABC=60^{\circ}
EAF=FAC=α\therefore \angle EAF=\angle FAC=\alphaACF=DCF=β\angle ACF=\angle DCF=\beta2α+2β=180ABC=18060=1202\alpha +2\beta =180^{\circ}-\angle ABC=180^{\circ}-60^{\circ}=120^{\circ}
α+β=60\therefore \alpha +\beta =60^{\circ}
AFC\triangle AFC中,AFC=180(CAF+ACF)=180(α+β)=18060=120\angle AFC=180^{\circ}-\left(\angle CAF+\angle ACF\right)=180^{\circ}-\left(\alpha +\beta \right)=180^{\circ}-60^{\circ}=120^{\circ}
故①说法正确,符合题意;
AD\because ADBAC\angle BAC的角平分线,不是三角形的中线,
BD\therefore BDCDCD不一定相等,故SABDS_{\triangle ABD}SADCS_{\triangle ADC}不一定相等,
故②说法错误,不符合题意;
CEABCE\bot AB,则CEA=CEB=90\angle CEA=\angle CEB=90^{\circ}
CE\because CE平分ACB\angle ACB
ACF=DCF=β\therefore \angle ACF=\angle DCF=\beta
ACE\triangle ACEBCE\triangle BCE中,
{CEA=CEBCE=CEACE=BCE\left\{\begin{array}{l}{∠CEA=∠CEB}\\{CE=CE}\\{∠ACE=∠BCE}\end{array}\right.
ACE\therefore \triangle ACEBCE(ASA)\triangle BCE\left(ASA\right)
AE=BE\therefore AE=BE
AB=2AE\therefore AB=2AE
故③说法正确,符合题意;
如图11,在ACAC边上取AG=AEAG=AE,连接FGFG

AD\because AD平分BAC\angle BAC
EAF=GAF\therefore \angle EAF=\angle GAF
AEF\triangle AEFAGF\triangle AGF中,
{AE=AGEAF=GAFAF=AF\left\{\begin{array}{l}{AE=AG}\\{∠EAF=∠GAF}\\{AF=AF}\end{array}\right.
AEF\therefore \triangle AEFAGF(ASA)\triangle AGF\left(ASA\right)
AFE=AFG\therefore \angle AFE=\angle AFGSAEF=SAGFS_{\triangle AEF}=S_{\triangle AGF}
AFC=120\because \angle AFC=120^{\circ}
AFE=CFD=60\therefore \angle AFE=\angle CFD=60^{\circ}
AFE=AFG=CFG=CFD=60\therefore \angle AFE=\angle AFG=\angle CFG=\angle CFD=60^{\circ}
CE\because CE平分ACB\angle ACB
ACF=DCF=β\therefore \angle ACF=\angle DCF=\beta
FDC\therefore \triangle FDCFGC(ASA)\triangle FGC\left(ASA\right)
CD=CG\therefore CD=CGSFDC=SFGCS_{\triangle FDC}=S_{\triangle FGC}
AC=AG+CG=AE+CD\therefore AC=AG+CG=AE+CD
故④说法正确,符合题意;
GGGMAFGM\bot AFMMGNCFGN\bot CFNN,如图22

AFG=CFG=60\because \angle AFG=\angle CFG=60^{\circ}
GM=GN\therefore GM=GN
SAEF=SAGF\because S_{\triangle AEF}=S_{\triangle AGF}SFDC=SFGCS_{\triangle FDC}=S_{\triangle FGC}
SAEFSFDC=SAGFSFGC=(12GMAF)(12GNFC)=AFFC\therefore {S}_{△AEF}:{S}_{△FDC}={S}_{△AGF}:{S}_{△FGC}=(\frac{1}{2}GM•AF):(\frac{1}{2}GN•FC)=AF:FC
故⑤说法正确,符合题意;
综上,说法正确的有①③④⑤,共44个.
故选:CC.

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