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八年级数学填空题一般
题目
已知在等腰ABC\triangle ABC中,AB=ACAB=AC,BC=m.BAC=30BC=m.\angle BAC=30^{\circ},点DD是直线BCBC上一点,连接ADAD,在ADAD的右侧作等腰ADE\triangle ADE,其中AD=AEAD=AE,EAD=30\angle EAD=30^{\circ},连接CECE,则(AE+33CE)(AE+\frac{\sqrt{3}}{3}CE)的最小值为______(用含mm的代数式表示).
知识点:三角形、全等三角形的判定章节:第13章 三角形 / 13.1 三角形的概念

答案与解析

答案

如图,在ABC\triangle ABC的下方作直线BGBG,使CBG=30\angle CBG=30^{\circ},过点DDDFBCDF\bot BCBGBGFF,过点BBBHACBH\bot ACHH,过点AAALBCAL\bot BCLL,交BHBHKK,连接CKCKAFAF

BAC=EAD=30\because \angle BAC=\angle EAD=30^{\circ}
EAC=BAD\therefore \angle EAC=\angle BAD
AB=AC\because AB=ACAE=ADAE=AD
BAD\therefore \triangle BADCAE(SAS)\triangle CAE\left(SAS\right)
ACE=B\therefore \angle ACE=\angle BBD=CEBD=CE
RtBDFRt\triangle BDF中,FBD=30\angle FBD=30^{\circ}
2DF=BF\therefore 2DF=BF
BF2DF2=BD2\because BF^{2}-DF^{2}=BD^{2}
(2DF)2DF2=BD2\therefore \left(2DF\right)^{2}-DF^{2}=BD^{2}
DF=33BD\therefore DF=\frac{\sqrt{3}}{3}BD
AE+33CE=AD+33BD=AD+DFAF\therefore AE+\frac{\sqrt{3}}{3}CE=AD+\frac{\sqrt{3}}{3}BD=AD+DF\geqslant AF
AB=AC\because AB=ACBAC=30\angle BAC=30^{\circ}
ABC=75\therefore \angle ABC=75^{\circ}
ABF\triangle ABF中,ABF=75+30=105\angle ABF=75^{\circ}+30^{\circ}=105^{\circ}为钝角,
AF>AB\therefore AF \gt AB
\therefore当点DD与点BB重合时,AE+33CE=AD+33BD=ABAE+\frac{\sqrt{3}}{3}CE=AD+\frac{\sqrt{3}}{3}BD=AB为最小值,
AB=AC=xAB=AC=x
RtABHRt\triangle ABH中,BAH=30\angle BAH=30^{\circ}
BH=12AB=12x\therefore BH=\frac{1}{2}AB=\frac{1}{2}x
AH=AB2BH2=32x\therefore AH=\sqrt{A{B}^{2}-B{H}^{2}}=\frac{\sqrt{3}}{2}x
CH=ACAH=232x\therefore CH=AC-AH=\frac{2-\sqrt{3}}{2}x
RtBCHRt\triangle BCH中,BH2+CH2=BC2BH^{2}+CH^{2}=BC^{2}
(12x)2+(232x)2=m2\therefore (\frac{1}{2}x)^{2}+(\frac{2-\sqrt{3}}{2}x)^{2}=m^{2}
整理得:x2=(2+3)m2=(6+22m)2x^{2}=(2+\sqrt{3})m^{2}=(\frac{\sqrt{6}+\sqrt{2}}{2}m)^{2}
x>0\because x \gt 0
x=6+22m\therefore x=\frac{\sqrt{6}+\sqrt{2}}{2}m
AB=6+22mAB=\frac{\sqrt{6}+\sqrt{2}}{2}m
AE+33CE\therefore AE+\frac{\sqrt{3}}{3}CE的最小值为6+22m\frac{\sqrt{6}+\sqrt{2}}{2}m
故答案为:6+22m\frac{\sqrt{6}+\sqrt{2}}{2}m.

解析

如图,在ABC\triangle ABC的下方作直线BGBG,使CBG=30\angle CBG=30^{\circ},过点DDDFBCDF\bot BCBGBGFF,过点BBBHACBH\bot ACHH,过点AAALBCAL\bot BCLL,交BHBHKK,连接CKCKAFAF

BAC=EAD=30\because \angle BAC=\angle EAD=30^{\circ}
EAC=BAD\therefore \angle EAC=\angle BAD
AB=AC\because AB=ACAE=ADAE=AD
BAD\therefore \triangle BADCAE(SAS)\triangle CAE\left(SAS\right)
ACE=B\therefore \angle ACE=\angle BBD=CEBD=CE
RtBDFRt\triangle BDF中,FBD=30\angle FBD=30^{\circ}
2DF=BF\therefore 2DF=BF
BF2DF2=BD2\because BF^{2}-DF^{2}=BD^{2}
(2DF)2DF2=BD2\therefore \left(2DF\right)^{2}-DF^{2}=BD^{2}
DF=33BD\therefore DF=\frac{\sqrt{3}}{3}BD
AE+33CE=AD+33BD=AD+DFAF\therefore AE+\frac{\sqrt{3}}{3}CE=AD+\frac{\sqrt{3}}{3}BD=AD+DF\geqslant AF
AB=AC\because AB=ACBAC=30\angle BAC=30^{\circ}
ABC=75\therefore \angle ABC=75^{\circ}
ABF\triangle ABF中,ABF=75+30=105\angle ABF=75^{\circ}+30^{\circ}=105^{\circ}为钝角,
AF>AB\therefore AF \gt AB
\therefore当点DD与点BB重合时,AE+33CE=AD+33BD=ABAE+\frac{\sqrt{3}}{3}CE=AD+\frac{\sqrt{3}}{3}BD=AB为最小值,
AB=AC=xAB=AC=x
RtABHRt\triangle ABH中,BAH=30\angle BAH=30^{\circ}
BH=12AB=12x\therefore BH=\frac{1}{2}AB=\frac{1}{2}x
AH=AB2BH2=32x\therefore AH=\sqrt{A{B}^{2}-B{H}^{2}}=\frac{\sqrt{3}}{2}x
CH=ACAH=232x\therefore CH=AC-AH=\frac{2-\sqrt{3}}{2}x
RtBCHRt\triangle BCH中,BH2+CH2=BC2BH^{2}+CH^{2}=BC^{2}
(12x)2+(232x)2=m2\therefore (\frac{1}{2}x)^{2}+(\frac{2-\sqrt{3}}{2}x)^{2}=m^{2}
整理得:x2=(2+3)m2=(6+22m)2x^{2}=(2+\sqrt{3})m^{2}=(\frac{\sqrt{6}+\sqrt{2}}{2}m)^{2}
x>0\because x \gt 0
x=6+22m\therefore x=\frac{\sqrt{6}+\sqrt{2}}{2}m
AB=6+22mAB=\frac{\sqrt{6}+\sqrt{2}}{2}m
AE+33CE\therefore AE+\frac{\sqrt{3}}{3}CE的最小值为6+22m\frac{\sqrt{6}+\sqrt{2}}{2}m
故答案为:6+22m\frac{\sqrt{6}+\sqrt{2}}{2}m.

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