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電子課本網(wǎng) 第132頁(yè)

第132頁(yè)

信息發(fā)布者:
證明?$: (1) $?∵  四邊形?$ A B C D $?是菱形?$, ∠A B C=60° ,$?
∴?$A B=B C=A D=C D, ∠A D C=∠A B C=60° \text {, }$?
 ∴?$\triangle {ADC} $?是等邊三角形,
∴?$A D=A C=A B=B C$?
 ∴?$\triangle A C B $?是等邊三角形,
∴?$∠{ACB}=∠{ACD}=60°, $?
∴?$∠{ACF}=120°, $?
∵?$∠{ADC}=∠{EDF}=60°, $?
∴?$∠{ADE}=∠{CDF}, $?
∵?$∠{EDF}+∠{ECF}+∠{DEC}+∠{DFC}=360°, $?
∴?$∠{DEC}+∠{DFC}=180°, $?
∵?$∠{DEC}+∠{AED}=180°, $?
∴?$∠{AED}=∠{DFC},$?
在?$ \triangle {ADE} $?和?$ \triangle {CDF} $?中,
?$\begin {cases}{∠A D E=∠C D F }\\{∠A E D=∠C F D }\\{A D=C D}\end {cases}$?
∴?$\triangle {ADE} \cong \triangle {CDF}({AAS}), $?
∴?${AE}={CF} ;$?
?$(2)$?過(guò)點(diǎn)?$B$?作?$ B H / / A C , $?交?$ A G $?的延長(zhǎng)線于點(diǎn)?$ {H} ,$?
 ∵?${BH}\ \mathrm {/} / {AC} ,$?
∴?$∠{H}=∠{GAE}, ∠{ABH}+∠{BAC}=180°, $?
∴?$∠{ABH}=120°=∠{ACF},$?
 ∵  點(diǎn)?$ {G} $?為?$ {BE} $?的中點(diǎn),
∴?$B G=G E \text {, }$?
在?$ \triangle {AGE} $?和?$ \triangle {HGB} $?中,
?$\begin {cases}{∠H=∠G A E }\\{∠B G H=∠A G E, }\\{B G=G E}\end {cases}$?
 ∴?$\triangle {AGE} \cong \triangle {HGB}({AAS}) ,$?
∴?$A E=B H=C F, \quad A G=G H=\frac{1}{2} A H \text {, }$?
在?$ \triangle {ABH} $?和?$ \triangle {ACF} $?中,
?$\begin {cases}{A B=A C }\\{∠A B H=∠A C F, }\\{B H=C F}\end {cases}$?
∴?$\triangle {ABH} \cong \triangle {ACF}({SAS}), $?
∴?${AF}={AH}, $?
∴?${AF}=2 {AG} .$?

解:?$(1)$?因?yàn)?$EF$?垂直平分?$BD,$?
所以?$BE= DE,$??$BF= DF,$??$BO= DO,$?
?$EF⊥BD,$?
因?yàn)?$AD// BC,$?
所以?$∠ADB=∠DBC,$?
在?$△DOE$?和?$△BOF $?中,
?$\begin {cases}{∠ADB=∠DBC } \\{DO=BO}\\{∠DOE=∠BOF} \end {cases}$?
所以?$△DOE≌△BOF (\mathrm {ASA}),$?
所以?$EO= FO,$?
所以四邊形?$DEBF$?是平行四邊形,
又因?yàn)?$EF⊥BD,$?
所以平行四邊形?$BEDF$?是菱形;
因?yàn)?$BE2= AE2+ AB2,$?
所以?$DE2=9+(9- DE)2$?
所以?$DE = 5\ \mathrm {cm},$?
所以菱形?$BEDF$?的邊長(zhǎng)為?$5\ \mathrm {cm}.$?
?$(2)$?因?yàn)榱庑?$BEDF$?的邊長(zhǎng)為?$5\ \mathrm {cm},$?
所以?$BE= DE= DF= BF =5\ \mathrm {cm},$?
所以?$AE= CF = 4\ \mathrm {cm},$?
因?yàn)辄c(diǎn)?$M$?自?$E→B→A→E$?停止,點(diǎn)?$N$?自?$F→C→D→F$?停止.
在運(yùn)動(dòng)過(guò)程中,已知點(diǎn)?$M$?的速度為?$5\ \mathrm {cm}/s,$?點(diǎn)?$N$?的速度為?$4\ \mathrm {cm}/s,$?
所以點(diǎn)?$M$?從點(diǎn)?$E$?到點(diǎn)?$B$?需要?$1s,$?點(diǎn)?$N$?從點(diǎn)?$F $?到點(diǎn)?$C$?需要?$1s$?
由題意可得:點(diǎn)?$M$?在?$BE$?上,點(diǎn)?$N$?在?$CF$?上時(shí),
點(diǎn)?$B,$??$D,$??$M,$??$N$?四個(gè)點(diǎn)不能構(gòu)成平行四邊形,
當(dāng)點(diǎn)?$M$?在?$AE$?上時(shí),點(diǎn)?$N$?在?$CD$?上時(shí),
點(diǎn)?$B,$??$D,$??$M,$??$N$?四個(gè)點(diǎn)不能構(gòu)成平行四邊形,
當(dāng)點(diǎn)?$M$?在?$AE$?上時(shí),點(diǎn)?$N$?在?$DF$?上時(shí),
點(diǎn)?$B,$??$D,$??$M,$??$ N$?四個(gè)點(diǎn)不能構(gòu)成平行四邊形,
所以只有點(diǎn)?$M$?在?$AB$?上時(shí),點(diǎn)?$N$?在?$CD$?上時(shí),
點(diǎn)?$B,$??$D,$??$ M,$??$ N$?四個(gè)點(diǎn)能構(gòu)成平行四邊形,
所以?$BM=MD$?
所以?$5t-5=7-4t$?
所以?$t=\frac {4}{3}$?