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八年级数学填空题一般
题目
如图,在ABC\triangle ABC中,ABC=45\angle ABC=45^{\circ},BAC=75\angle BAC=75^{\circ},AC=2AC=2,点EE与点DD分别在射线BCBC与射线ADAD上,且AD=BEAD=BE,则AE+BDAE+BD的最小值为______,AE+EDAE+ED的最小值为______.
知识点:三角形的三边关系、全等三角形的性质、全等三角形的判定章节:第一章 三角形 / 1.1 认识三角形

答案与解析

答案

如图所示,过AAAFBCAF\bot BCBCBC的于FF

ABC=45\because \angle ABC=45^{\circ}BAC=75\angle BAC=75^{\circ}
ACB=1804575=60\therefore \angle ACB=180^{\circ}-45^{\circ}-75^{\circ}=60^{\circ}
CAF=30\therefore \angle CAF=30^{\circ}ABC=BAF=45\angle ABC=\angle BAF=45^{\circ}
AC=2\because AC=2
CF=12AC=1\therefore CF=\frac{1}{2}AC=1AF=BF=AC2CF2=3AF=BF=\sqrt{A{C}^{2}-C{F}^{2}}=\sqrt{3}
AB=AF2+BF2=6\therefore AB=\sqrt{A{F}^{2}+B{F}^{2}}=\sqrt{6}
如图所示,作MAD=45\angle MAD=45^{\circ}AM=ABAM=AB,连接DMDMBMBM

AB=AM\because AB=AMABE=MAD=45\angle ABE=\angle MAD=45^{\circ}BE=ADBE=AD
ABE\therefore \triangle ABEMAD(SAS)\triangle MAD\left(SAS\right)
AE=DM\therefore AE=DM
BD+AE=BD+DMBM\therefore BD+AE=BD+DM\geqslant BM
DDBMBM上时,BD+AEBD+AE取得最小值,如图所示,过点MMMNABMN\bot ABBABA的延长线于点NN

BAD=75\because \angle BAD=75^{\circ}DAM=45\angle DAM=45^{\circ}
NAM=60\therefore \angle NAM=60^{\circ}AMN=30\angle AMN=30^{\circ}
AB=AM\because AB=AM
ABM=30\therefore \angle ABM=30^{\circ}
AM=AB=6\because AM=AB=\sqrt{6}
RtANMRt\triangle ANM中,AN=12AM=62AN=\frac{1}{2}AM=\frac{\sqrt{6}}{2}
MN=3AN=322\therefore MN=\sqrt{3}AN=\frac{3\sqrt{2}}{2}
BM=2MN=32\therefore BM=2MN=3\sqrt{2},即AE+BDAE+BD的最小值为323\sqrt{2}
如图所示,作AA关于BMBM的对称点JJ,连接AJAJBJBJMJMJEJEJDJDJ

AB=AM\because AB=AMBAM=120\angle BAM=120^{\circ}AB=AMAB=AMABM=JBM=30\angle ABM=\angle JBM=30^{\circ}
ABJ=60\therefore \angle ABJ=60^{\circ}
\because是对称,
BA=BJ\therefore BA=BJ
ABJ\therefore \triangle ABJAMJ\triangle AMJ都是等边三角形,
ABE\because \triangle ABEMAD\triangle MAD
BAE=AMD\therefore \angle BAE=\angle AMD,则EAJ=DMJ\angle EAJ=\angle DMJ
AJ=JM\because AJ=JMAE=MDAE=MD
EAJ\therefore \triangle EAJDMJ\triangle DMJ
EJA=DJM\therefore \angle EJA=\angle DJMEJ=DJEJ=DJ
EJD=AJM=60\therefore \angle EJD=\angle AJM=60^{\circ}
EDJ\therefore \triangle EDJ是等边三角形,
AE+ED=AE+EJAJ\therefore AE+ED=AE+EJ\geqslant AJ
\thereforeEEAJAJ上时,AE+ED=AJAE+ED=AJ,如图所示,

此时AE+EDAE+ED取得最小值,最小值AJ=AB=6AJ=AB=\sqrt{6}
故答案为:323\sqrt{2}6\sqrt{6}.

解析

如图所示,过AAAFBCAF\bot BCBCBC的于FF

ABC=45\because \angle ABC=45^{\circ}BAC=75\angle BAC=75^{\circ}
ACB=1804575=60\therefore \angle ACB=180^{\circ}-45^{\circ}-75^{\circ}=60^{\circ}
CAF=30\therefore \angle CAF=30^{\circ}ABC=BAF=45\angle ABC=\angle BAF=45^{\circ}
AC=2\because AC=2
CF=12AC=1\therefore CF=\frac{1}{2}AC=1AF=BF=AC2CF2=3AF=BF=\sqrt{A{C}^{2}-C{F}^{2}}=\sqrt{3}
AB=AF2+BF2=6\therefore AB=\sqrt{A{F}^{2}+B{F}^{2}}=\sqrt{6}
如图所示,作MAD=45\angle MAD=45^{\circ}AM=ABAM=AB,连接DMDMBMBM

AB=AM\because AB=AMABE=MAD=45\angle ABE=\angle MAD=45^{\circ}BE=ADBE=AD
ABE\therefore \triangle ABEMAD(SAS)\triangle MAD\left(SAS\right)
AE=DM\therefore AE=DM
BD+AE=BD+DMBM\therefore BD+AE=BD+DM\geqslant BM
DDBMBM上时,BD+AEBD+AE取得最小值,如图所示,过点MMMNABMN\bot ABBABA的延长线于点NN

BAD=75\because \angle BAD=75^{\circ}DAM=45\angle DAM=45^{\circ}
NAM=60\therefore \angle NAM=60^{\circ}AMN=30\angle AMN=30^{\circ}
AB=AM\because AB=AM
ABM=30\therefore \angle ABM=30^{\circ}
AM=AB=6\because AM=AB=\sqrt{6}
RtANMRt\triangle ANM中,AN=12AM=62AN=\frac{1}{2}AM=\frac{\sqrt{6}}{2}
MN=3AN=322\therefore MN=\sqrt{3}AN=\frac{3\sqrt{2}}{2}
BM=2MN=32\therefore BM=2MN=3\sqrt{2},即AE+BDAE+BD的最小值为323\sqrt{2}
如图所示,作AA关于BMBM的对称点JJ,连接AJAJBJBJMJMJEJEJDJDJ

AB=AM\because AB=AMBAM=120\angle BAM=120^{\circ}AB=AMAB=AMABM=JBM=30\angle ABM=\angle JBM=30^{\circ}
ABJ=60\therefore \angle ABJ=60^{\circ}
\because是对称,
BA=BJ\therefore BA=BJ
ABJ\therefore \triangle ABJAMJ\triangle AMJ都是等边三角形,
ABE\because \triangle ABEMAD\triangle MAD
BAE=AMD\therefore \angle BAE=\angle AMD,则EAJ=DMJ\angle EAJ=\angle DMJ
AJ=JM\because AJ=JMAE=MDAE=MD
EAJ\therefore \triangle EAJDMJ\triangle DMJ
EJA=DJM\therefore \angle EJA=\angle DJMEJ=DJEJ=DJ
EJD=AJM=60\therefore \angle EJD=\angle AJM=60^{\circ}
EDJ\therefore \triangle EDJ是等边三角形,
AE+ED=AE+EJAJ\therefore AE+ED=AE+EJ\geqslant AJ
\thereforeEEAJAJ上时,AE+ED=AJAE+ED=AJ,如图所示,

此时AE+EDAE+ED取得最小值,最小值AJ=AB=6AJ=AB=\sqrt{6}
故答案为:323\sqrt{2}6\sqrt{6}.

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