(精品)平行四邊形的性質(zhì)1、2 (4)
,*,單擊此處編輯母版文本樣式,第二級(jí),單擊此處編輯母版標(biāo)題樣式,19.2,平行四邊形,(,一),01,03,02,04,認(rèn)識(shí)平行四邊形,添加標(biāo)題,平行四邊形,的性質(zhì),平行四邊形性質(zhì),的運(yùn)用,課堂練習(xí),觀察,思考,觀察下列圖案,想一想它們都是什么形狀?,有何特點(diǎn)?,每人手里有兩個(gè)全等的三角形,拿這兩個(gè)三角形去拼四邊形,看誰(shuí)拼出的四邊形多?,動(dòng)手操作,平行四邊形的定義:,兩組對(duì)邊分別平行的四邊形叫做,平行四邊形,.,觀察圖形,說(shuō)出各四邊形中的邊的位置,有何特征?,兩組對(duì)邊,都不平行,一組對(duì)邊平行,另,一組對(duì)邊不平行,兩組對(duì)邊,分別平行,如圖,四邊形,ABCD,是平行四邊形,,讀作:平行四邊形,ABCD,,,其中,,AD,與,BC,叫對(duì)邊,,AB,與,CD,叫對(duì)邊,,表示:,ABCD,ABCD,的四個(gè)頂點(diǎn):點(diǎn),A,、點(diǎn),B,、點(diǎn),C,、點(diǎn),D,,,ABCD,的四條邊:,AB,、,BC,、,CD,、,AD,,,ABCD,的四個(gè)內(nèi)角:,A,、,B,、,C,、,D,,,其中,,A,與,C,叫對(duì)角,,B,與,D,叫對(duì)角,,認(rèn)識(shí)平行四邊形,平行四邊形的對(duì)邊平行,除此以外,平行四邊形中,邊、角還有什么性質(zhì)呢?,圖中,,AD,BC,,,AB,DC,,,A,+,B,180,,,A,+,D,180,,,B,+,C,180,,,C,+,D,180,,,觀察,復(fù),制一個(gè)平行四邊形使它與原平行四邊形重合,再用大頭針把對(duì)角線的交點(diǎn),固定,把上面的平行四邊形繞點(diǎn),旋轉(zhuǎn),1,80,,它與原來(lái)的四邊形,ABCD,重合嗎?,探究證明:,已知:如圖,四邊形,ABCD,中,,AB,DC,,,AD,BC,,,求證:,(,1,),AB,DC,,,AD,BC,;,(,2,),DAB,DCA,,,B,D,,,證明:連接,AC,,,(1),AB,DC,,,AD,BC,,,BAC,DCA,,,BCA,DAC,,,在,ABC,和,CDA,中,,ABC,CDA,(,ASA,),AB,DC,,,AD,BC,;,(2),由,(1),知:,ABC,CDA,,,B,D,,,DAB,BAC,+,DAC,DCA,+,BCA,DCB,.,結(jié)論:由此得到平行四邊形的性質(zhì):,性質(zhì),1,:平行四邊形的對(duì)邊相等,.,性質(zhì),2,:平行四邊形的對(duì)角相等,.,由此可以看出:如下圖,,四邊形,ABCD,是平行四邊形,,AB,DC,,,AD,BC,;,A,C,,,B,D,,,例題講解,例,1,已知:如圖,,ABCD,中,,BE,平分,ABC,交,AD,于點(diǎn),E,,,(1),如果,AE,2,,求,CD,的長(zhǎng);,(2),如果,AEB,40,,求,C,的度數(shù),.,解:,(1),四邊形,ABCD,是平行四邊形,,BE,平分,ABC,,,AD,BC,,,2,3,1,2,,,1,3,,,AB,AE,2,,,又,CD,AB,,,CD,2,;,(,2,),由,(,1,),知:,1,3,40,,,A,180,1,3,100,,,又,C,A,,,C,100.,A,B,C,D,隨堂練習(xí),1.,如圖,如果,AB CD,,,AD BC,那么四邊形,ABCD,是,_,四邊形,記作:,_,隨堂練習(xí),A,B,C,D,3.,在,ABCD,中,已知,A,60,,則,B=_,,,C=_,,,D=_,2,、,4.,在,ABCD,中,A,B=60,則,A=_,B=_,C=_,D=_,A,B,C,D,4,、在,ABCD,中,已知,AB,a,,,BC,b,,,則這個(gè)平行四邊形的周長(zhǎng)為,_,5,、如果,ABCD,周長(zhǎng)為,35cm,,已知,AB:BC,3:4,,那么,AB=_cm,BC=_cm.,解:取,AD,中點(diǎn),F,,連接,EF,,,BC,2,AB,,,AB,BE,CD,CE,,,又,AB,EF,CD,,,AED,EAB,+,EDC,AEB,DEC,AED,+,AEB,+,DEC,180,,,AEC,90,,,AE,ED,.,3,在,ABCD,中,,BC,2,AB,,點(diǎn),E,為邊,BC,的,中點(diǎn)求證:,AE,ED,F,則,AB,EF,CD,.,(,2,),平行四邊形的性質(zhì)及應(yīng)用;,小結(jié)與反思,(,1,),認(rèn)識(shí)平行四邊形及平行四邊形的定義,;,本節(jié)課你學(xué)習(xí)了哪些主要內(nèi)容,與同伴交流,.,兩組對(duì)邊分別平行的四邊形叫做平行四邊形,.,性質(zhì),1,:平行四邊形的對(duì)邊相等;,性質(zhì),2,:平行四邊形的對(duì)角相等;,