We're going to play ________ computer games and do ________ exercise. A . fewer, more B . less, more C . fewer, less D . less, fewer 答案:A如图,⊙O的直径AB垂直弦CD于点E,点F在AB的延长线上,且∠BCF=∠A.(1)求证:直线CF是⊙O的切线;(2)若⊙O的半径为5,DB=4.求sin∠D的值.
(1)证明见解析;(2).
【解析】
试题分析:(1)连接OC,由OA=OA可知∠ACO=∠A,再根据∠FCB=∠A可知∠ACO=∠FCB,由于AB是⊙O的直径,所以∠ACO+∠OCB=90°故∠FCB+∠OCB=90°故可得出结论;
(2)由AB是⊙O的直径,CD⊥AB可知
试题解析: (1)连接OC,
∵OA=OC,
∴∠ACO=∠A,
又∵∠FCB=...