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29.3,切线的性质和判定,第,1,课时,切线的性质,切线的性质在求边、角问题中的应用,切线的性质在判断图形形状中的应用,利用切线的性质解与特殊四边形综合问题,利用圆的切线性质求角的大小,1,2,3,4,12,【,中考,常德,】如图,已知,AB,是,O,的直径,,CD,与,O,相切,于,C,,,BE,CO,.,(1),求证:,BC,是,ABE,的平分线;,(2),若,DC,8,,,O,的半径,OA,6,,求,CE,的长,(1),证明:,DE,是,O,的切线,,OC,DE,.,BE,CO,,,OCB,CBE,.,OC,OB,,,OCB,OBC,.,CBE,OBC,.,BC,平分,ABE,.,(2),解:,在,Rt,CDO,中,,DC,8,,,OC,OA,6,,,OD,10.,OC,BE,,, . .,CE,4.8.,13,【,中考,温州,】如图,在,ABC,中,,AC,BC,,,ACB,90,,,O,(,圆心,O,在,ABC,内部,),经过,B,,,C,两点,,交,AB,于点,E,,过点,E,作,O,的切线交,AC,于点,F,.,延长,CO,交,AB,于点,G,,作,ED,AC,交,CG,于点,D.,(1),求证:四边形,CDEF,是平行四边形;,(2),若,BC,3,,,tan,DEF,2,,求,BG,的值,(1),证明:,如图,连接,CE,.,在,ABC,中,,AC,BC,,,ACB,90,,,B,45.,COE,90.,CEO,ECO,45.,EF,是,O,的切线,,FEO,90.,FEC,45.,FEC,ECO,.,EF,CG,.,又,ED,AC,,,四边形,CDEF,是平行四边形,(2),解:,如图,过,G,作,GM,BC,于,M,,,GMB,是等腰直角三角形,,MB,GM,.,四边形,CDEF,是平行四边形,,FCD,FED,.,ACD,GCB,GCB,CGM,90,,,CGM,ACD,.,CGM,DEF,.,tan,DEF,2,,,tan,CGM,2.,CM,2,GM,,,又,BC,CM,BM,2,GM,GM,3,,,GM,1.,BG,GM,.,14,【,中考,昆明,】如图,,AB,是,O,的直径,,BAC,90,,四边形,EBOC,是平行四边形,,EB,交,O,于,点,D,,连接,CD,并延长交,AB,的延长线于点,F,.,(1),求证:,CF,是,O,的切线;,(2),若,F,30,,,EB,4,,求图中阴影部分的面,积,(,结果保留根号和,),(1),证明:,如图,连接,OD,.,四边形,EBOC,是平行四边形,,OC,BE,.,AOC,OBE,,,COD,ODB,.,OB,OD,,,OBD,ODB,.,DOC,AOC,.,在,COD,和,COA,中,,COD,COA,.,CDO,CAO,90.,圆心,O,到,CF,的距离等于,O,的半径,CF,是,O,的切线,(2),解:,F,30,,,ODF,90,,,DOF,60,,,AOD,120.,OD,OB,,,OBD,是等边三角形,,BDO,DBO,60,,,OB,BD,.,易证,EDC,ECD,30,,,ED,EC,.,又,四边形,EBOC,是平行四边形,,EC,ED,BO,DB,.,EB,4,,,OB,OD,OA,2.,AOC,COD,,,AOC,60.,在,Rt,AOC,中,,OAC,90,,,AOC,60,,,OCA,30,,,OC,4.,AC,2 .,S,阴影,2,S,AOC,S,扇形,OAD,2 22,2,2,4,.,15,【,中考,天津,】已知,AB,是,O,的直径,,AT,是,O,的,切线,,ABT,50,,,BT,交,O,于点,C,,,E,是,AB,上一点,延长,CE,交,O,于点,D.,(1),如图,,求,T,和,CDB,的大小;,(2),如图,,当,BE,BC,时,求,CDO,的大小,解:,(1),如图,,连接,AC,,,AT,是,O,的切线,,AB,是,O,的直径,,TAB,90.,ABT,50,,,T,90,ABT,40.,由,AB,是,O,的直径,得,ACB,90.,CAB,90,ABC,40.,CDB,CAB,40.,(2),如图,,连接,AD,,,在,BCE,中,,BE,BC,,,EBC,50,,,BCE,BEC,65.,BAD,BCD,65.,OA,OD,,,ODA,OAD,65.,ADC,ABC,50,,,CDO,ODA,ADC,65,50,15.,
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