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八年级数学填空题一般
题目
如图,在ABC\triangle ABC中,ACB=60\angle ACB=60^{\circ},BC=6BC=6,分别以ABAB,ACAC为边在ABC\triangle ABC外作等边ABD\triangle ABD和等边ACE\triangle ACE,连结BEBE,CDCD.
(1)(1)BEC=24\angle BEC=24^{\circ},则CBE=______\angle CBE= \_\_\_\_\_\_^{\circ}
(2)(2)AC=8AC=8,则CDCD的长为______.
知识点:三角形的中位线定理、全等三角形的性质、全等三角形的判定、等边三角形的性质、三角形中位线定理的证明章节:第5章 平行四边形 / 5.3 三角形的中位线

答案与解析

答案

(1)ACE\left(1\right)\because \triangle ACE是等边三角形,
ACE=60=ACB\therefore \angle ACE=60^{\circ}=\angle ACB
BCE=120\therefore \angle BCE=120^{\circ}
BEC=24\because \angle BEC=24^{\circ}
CBE=180BCEBEC=18012024=36\therefore \angle CBE=180^{\circ}-\angle BCE-\angle BEC=180^{\circ}-120^{\circ}-24^{\circ}=36^{\circ}
故答案为:3636
(2)ABD(2)\because \triangle ABDACE\triangle ACE是等边三角形,
AD=AB\therefore AD=ABAE=AC=CE=4AE=AC=CE=4DAB=CAE=60\angle DAB=\angle CAE=60^{\circ}
DAC=BAE\therefore \angle DAC=\angle BAE
DAC\triangle DACBAE\triangle BAE中,
{DA=BADAC=BAEAC=AE\left\{\begin{array}{l}{DA=BA}\\{∠DAC=∠BAE}\\{AC=AE}\end{array}\right.
DAC\therefore \triangle DACBAE(SAS)\triangle BAE\left(SAS\right)
CD=BE\therefore CD=BE
过点EEEFBCEF\bot BC于点FF
BCE=120\because \angle BCE=120^{\circ}
CEF=BCEF=12090=30\therefore \angle CEF=\angle BCE-\angle F=120^{\circ}-90^{\circ}=30^{\circ}
CF=12CE=4\therefore CF=\frac{1}{2}CE=4
EF=CE2CF2=43\therefore EF=\sqrt{{CE}^{2}{-CF}^{2}}=4\sqrt{3}
BF=BC+CF=6+4=10BF=BC+CF=6+4=10
CD=BE=EF2+BF2=237\therefore CD=BE=\sqrt{{EF}^{2}{+BF}^{2}}=2\sqrt{37}
故答案为:(1)36\left(1\right)36^{\circ},(2)2372\sqrt{37}.

解析

(1)ACE\left(1\right)\because \triangle ACE是等边三角形,
ACE=60=ACB\therefore \angle ACE=60^{\circ}=\angle ACB
BCE=120\therefore \angle BCE=120^{\circ}
BEC=24\because \angle BEC=24^{\circ}
CBE=180BCEBEC=18012024=36\therefore \angle CBE=180^{\circ}-\angle BCE-\angle BEC=180^{\circ}-120^{\circ}-24^{\circ}=36^{\circ}
故答案为:3636
(2)ABD(2)\because \triangle ABDACE\triangle ACE是等边三角形,
AD=AB\therefore AD=ABAE=AC=CE=4AE=AC=CE=4DAB=CAE=60\angle DAB=\angle CAE=60^{\circ}
DAC=BAE\therefore \angle DAC=\angle BAE
DAC\triangle DACBAE\triangle BAE中,
{DA=BADAC=BAEAC=AE\left\{\begin{array}{l}{DA=BA}\\{∠DAC=∠BAE}\\{AC=AE}\end{array}\right.
DAC\therefore \triangle DACBAE(SAS)\triangle BAE\left(SAS\right)
CD=BE\therefore CD=BE
过点EEEFBCEF\bot BC于点FF
BCE=120\because \angle BCE=120^{\circ}
CEF=BCEF=12090=30\therefore \angle CEF=\angle BCE-\angle F=120^{\circ}-90^{\circ}=30^{\circ}
CF=12CE=4\therefore CF=\frac{1}{2}CE=4
EF=CE2CF2=43\therefore EF=\sqrt{{CE}^{2}{-CF}^{2}}=4\sqrt{3}
BF=BC+CF=6+4=10BF=BC+CF=6+4=10
CD=BE=EF2+BF2=237\therefore CD=BE=\sqrt{{EF}^{2}{+BF}^{2}}=2\sqrt{37}
故答案为:(1)36\left(1\right)36^{\circ},(2)2372\sqrt{37}.

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