摘要：28．如图(18).在平面直角坐标系中.的边在轴上.且.以为直径的圆过点．若点的坐标为..A.B两点的横坐标.是关于的方程的两根． (1)求.的值, (2)若平分线所在的直线交轴于点.试求直线对应的一次函数解析式, (3)过点任作一直线分别交射线.(点除外)于点.．则的是否为定值?若是.求出该定值,若不是.请说明理由． 28．解:(1)以为直径的圆过点. .而点的坐标为. 由易知. .····································································································· 1分 即:.解之得:或． .. 即．···································································································· 2分 由根与系数关系有: . 解之.．································································································ 4分 .过点作.交于点. 易知.且. 在中.易得.··········· 5分 . . 又.有. .······································································································· 6分 . 则.即.························································································ 7分 易求得直线对应的一次函数解析式为:．··················································· 8分 解法二:过作于.于. 由. 求得．········································································································ 5分 又. 求得．····························································································· 7分 即. 易求得直线对应的一次函数解析式为:．··················································· 8分 (3)过点作于.于． 为的平分线.． 由.有········································································· 9分 由.有······································································ 10分 .··············································································· 11分 即．···················································································· 12分

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