题目

如图，PA，PB是⊙O的切线，A，B为切点，AC是⊙O的直径，AC，PB的延长线相交于点D.(1)若∠1＝20°，求∠APB的度数．(2)当∠1为多少度时，OP＝OD？并说明理由． 答案：【答案】（1）40°；（2）当∠1＝30°时，OP＝OD.【解析】（1）首先证明PA=PB，求出∠PAB，∠PBA的度数即可解决问题．（2）当∠1=30°时，OP=OD．只要证明∠OPD=∠D=30°即可．解：(1)∵PA是⊙O的切线，∴PA⊥OA，∴∠BAP＝90°－∠1＝70°.又∵PA，PB是⊙O的切线，∴PA＝PB，∴∠ABP＝∠BAP＝70°.∴∠APB＝180°－70°×2＝4The fish _______ for some time. You’d better cook them at once.A. has died B. has been deadC. have died D. have been dead