题目内容
(本小题满分9分)已知⊙与⊙相交于、两点,点在⊙上,为⊙上一点(不与,,重合),直线与⊙交于另一点。
(1)如图(8),若是⊙的直径,求证:;
(2)如图(9),若是⊙外一点,求证:;
(3)如图(10),若是⊙内一点,判断(2)中的结论是否成立。
(1)如图(8),若是⊙的直径,求证:;
(2)如图(9),若是⊙外一点,求证:;
(3)如图(10),若是⊙内一点,判断(2)中的结论是否成立。
证明:(1)如图(一),连接,
∵为⊙的直径 ∴
∴为⊙的直径 ∴在上
又,为的中点
∴△是以为底边的等腰三角形
∴····················································································· (3分)
(2)如图(二),连接,并延长交⊙与点,连
∵四边形内接于⊙ ∴
又∵ ∴
∴
又为⊙的直径 ∴
∴···················································································· (3分)
(3)如图(三),连接,并延长交⊙与点,连
∵ 又
∴
∴ 又
∴···················································································· (3分)
∵为⊙的直径 ∴
∴为⊙的直径 ∴在上
又,为的中点
∴△是以为底边的等腰三角形
∴····················································································· (3分)
(2)如图(二),连接,并延长交⊙与点,连
∵四边形内接于⊙ ∴
又∵ ∴
∴
又为⊙的直径 ∴
∴···················································································· (3分)
(3)如图(三),连接,并延长交⊙与点,连
∵ 又
∴
∴ 又
∴···················································································· (3分)
略
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