如图，C是∠AOB内部一点，D是∠AOB外部一点，在内部求作一点P，使PC=PD，并且使P点到∠AOB两边距离相等（保留作图痕迹）．
解：如图所示：点P即为所求．
— Why did she ______ your invitation?
— Because she had an important meeting to attend that day.
A →①Cl2,光照 （一氯环己烷） →② →③Br2的CCl4溶液 B →④
（一氯环己烷） 
B 
