如图,在△ABC中,以AB为直径的⊙O分别交AC、BC于点D、E,点F在AC的延长线上,且AC=CF,∠CBF=∠CFB.(1
如图,在△ABC中,以AB为直径的⊙O分别交AC、BC于点D、E,点F在AC的延长线上,且AC=CF,∠CBF=∠CFB.(1)求证:直线BF是⊙O的切线;(2)若点D,...
如图,在△ABC中,以AB为直径的⊙O分别交AC、BC于点D、E,点F在AC的延长线上,且AC=CF,∠CBF=∠CFB.(1)求证:直线BF是⊙O的切线;(2)若点D,点E分别是弧AB的三等分点,当AD=5时,求BF的长;(3)填空:在(2)的条件下,如果以点C为圆心,r为半径的圆上总存在不同的两点到点O的距离为5,则r的取值范围为 .
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(1)证明见解析;(2)BF=10 ![](https://iknow-pic.cdn.bcebos.com/024f78f0f736afc3575ccdd7b019ebc4b745122b?x-bce-process=image%2Fresize%2Cm_lfit%2Cw_600%2Ch_800%2Climit_1%2Fquality%2Cq_85%2Fformat%2Cf_auto) ;(3)5 ![](https://iknow-pic.cdn.bcebos.com/024f78f0f736afc3575ccdd7b019ebc4b745122b?x-bce-process=image%2Fresize%2Cm_lfit%2Cw_600%2Ch_800%2Climit_1%2Fquality%2Cq_85%2Fformat%2Cf_auto) ?5<r<5+5. |
试题分析:(1)欲证明直线BF是⊙O的切线,只需证明AB⊥BF; (2)根据圆心角、弧、弦间的关系,等边三角形的判定证得△AOD是等边三角形,所以在Rt△ABF中,∠ABF=90°,∠OAD=60°,AB=10,则利于∠A的正切三角函数的定义来求BF边的长度; (3)根据已知条件知⊙O与⊙C相交. (1)证明:如图,∵∠CBF=∠CFB, ∴CB=CF. 又∵AC=CF, ∴CB= ![](https://iknow-pic.cdn.bcebos.com/aa18972bd40735fafea0bc0b9d510fb30f24082b?x-bce-process=image%2Fresize%2Cm_lfit%2Cw_600%2Ch_800%2Climit_1%2Fquality%2Cq_85%2Fformat%2Cf_auto) AF, ∴△ABF是直角三角形, ∴∠ABF=90°,即AB⊥BF. 又∵AB是直径, ∴直线BF是⊙O的切线. (2)解:如图,连接DO,EO, ∵点D,点E分别是弧AB的三等分点, ∴∠AOD=60°. 又∵OA=OD, ∴△AOD是等边三角形, ∴OA=AD=OD=5,∠OAD=60°, ∴AB=10. ∴在Rt△ABF中,∠ABF=90°,BF=AB?tan60°=10 ![](https://iknow-pic.cdn.bcebos.com/024f78f0f736afc3575ccdd7b019ebc4b745122b?x-bce-process=image%2Fresize%2Cm_lfit%2Cw_600%2Ch_800%2Climit_1%2Fquality%2Cq_85%2Fformat%2Cf_auto) ,即BF=10 ![](https://iknow-pic.cdn.bcebos.com/024f78f0f736afc3575ccdd7b019ebc4b745122b?x-bce-process=image%2Fresize%2Cm_lfit%2Cw_600%2Ch_800%2Climit_1%2Fquality%2Cq_85%2Fformat%2Cf_auto) ; (3)如图,连接OC.则OC是Rt△ABF的中位线, ∵由(2)知,BF=10 ![](https://iknow-pic.cdn.bcebos.com/024f78f0f736afc3575ccdd7b019ebc4b745122b?x-bce-process=image%2Fresize%2Cm_lfit%2Cw_600%2Ch_800%2Climit_1%2Fquality%2Cq_85%2Fformat%2Cf_auto) , ∴圆心距OC=5 ![](https://iknow-pic.cdn.bcebos.com/024f78f0f736afc3575ccdd7b019ebc4b745122b?x-bce-process=image%2Fresize%2Cm_lfit%2Cw_600%2Ch_800%2Climit_1%2Fquality%2Cq_85%2Fformat%2Cf_auto) , ∵⊙O半径OA=5. ∴5 ![](https://iknow-pic.cdn.bcebos.com/024f78f0f736afc3575ccdd7b019ebc4b745122b?x-bce-process=image%2Fresize%2Cm_lfit%2Cw_600%2Ch_800%2Climit_1%2Fquality%2Cq_85%2Fformat%2Cf_auto) ?5<r<5 ![](https://iknow-pic.cdn.bcebos.com/024f78f0f736afc3575ccdd7b019ebc4b745122b?x-bce-process=image%2Fresize%2Cm_lfit%2Cw_600%2Ch_800%2Climit_1%2Fquality%2Cq_85%2Fformat%2Cf_auto) +5. 故填:5 ![](https://iknow-pic.cdn.bcebos.com/024f78f0f736afc3575ccdd7b019ebc4b745122b?x-bce-process=image%2Fresize%2Cm_lfit%2Cw_600%2Ch_800%2Climit_1%2Fquality%2Cq_85%2Fformat%2Cf_auto) ?5<r<5+5. |
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