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八年级数学选择题一般
题目
如图,在RtABCRt\triangle ABC中,ABC=90\angle ABC=90^{\circ},以ACAC为边,作ACD\triangle ACD,满足AD=ACAD=AC,EEBCBC上一点,连接AEAE,2BAE=CAD2\angle BAE=\angle CAD,连接DEDE,下列结论中正确的有( )
ACDEAC\bot DE;②ADE=ACB\angle ADE=\angle ACB;③若CDCDABAB,则AEADAE\bot AD;④DE=CE+2BEDE=CE+2BE.
A.
①②③
B.
②③④
C.
②③
D.
①②④
知识点:角的运算、三角形内角和定理、全等三角形的性质、全等三角形的判定、等腰三角形的性质、勾股定理、旋转的性质章节:第13章 三角形 / 13.1 三角形的概念

答案与解析

答案

B

解析

如图,延长EBEBGG,使BE=BGBE=BG,设ACACDEDE交于点MM
ABC=90\because \angle ABC=90^{\circ}
ABGE\therefore AB\bot GE
AB\therefore AB垂直平分GEGE
AG=AE\therefore AG=AEGAB=BAE=12DAC\angle GAB=\angle BAE=\frac{1}{2}\angle DAC
BAE=12GAE\because \angle BAE=\frac{1}{2}\angle GAE
GAE=CAD\therefore \angle GAE=\angle CAD
GAE+EAC=CAD+EAC\therefore \angle GAE+\angle EAC=\angle CAD+\angle EAC
GAC=EAD\therefore \angle GAC=\angle EAD
GAC\triangle GACEAD\triangle EAD中,
{AG=AEGAC=EADAC=AD\left\{\begin{array}{l}{AG=AE}\\{∠GAC=∠EAD}\\{AC=AD}\end{array}\right.
GAC\therefore \triangle GACEAD(SAS)\triangle EAD\left(SAS\right)
G=AED\therefore \angle G=\angle AEDACB=ADE\angle ACB=\angle ADE
\therefore②是正确的;
AG=AE\because AG=AE
G=AEG=AED\therefore \angle G=\angle AEG=\angle AED
AE\therefore AE平分BED\angle BED
BAE=EAC\angle BAE=\angle EAC时,AME=ABE=90\angle AME=\angle ABE=90^{\circ},则ACDEAC\bot DE
BAEEAC\angle BAE\neq \angle EAC时,AMEABE\angle AME\neq \angle ABE,则无法说明ACDEAC\bot DE
\therefore①是不正确的;
BAE=x\angle BAE=x,则CAD=2x\angle CAD=2x
ACD=ADC=180°2x2=90x\therefore \angle ACD=\angle ADC=\frac{180°-2x}{2}=90^{\circ}-x
AB\because ABCDCD
BAC=ACD=90x\therefore \angle BAC=\angle ACD=90^{\circ}-x
CAE=BACEAB=90xx=902x\therefore \angle CAE=\angle BAC-\angle EAB=90^{\circ}-x-x=90^{\circ}-2x
DAE=CAE+DAC=902x+2x=90\therefore \angle DAE=\angle CAE+\angle DAC=90^{\circ}-2x+2x=90^{\circ}
AEAD\therefore AE\bot AD
\therefore③是正确的;
GAC\because \triangle GACEAD\triangle EAD
CG=DE\therefore CG=DE
CG=CE+GE=CE+2BE\because CG=CE+GE=CE+2BE
DE=CE+2BE\therefore DE=CE+2BE
\therefore④是正确的,
故选:BB.

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