题目

Mr. Brown was going away for a week．Before he left，（A）he said to his son，“If anyone asks for me，you can tell him that your father has been out for doing something，and will be back in a week．Then be sure to ask him to sit down for a cup of tea．”     “Ok，Dad，”said his son．But he was afraid his son couldn’t remember this，he wrote these words down on a piece of paper and gave it to him．（B）His son put it into his small pocket，took it out and looked at it every now and then． Four days passed，but no one came to see his father．The boy thought that t著名数学教育家G．波利亚，有句名言：“发现问题比解决问题更重要”，这句话启发我们：要想学好数学，就需要观察，发现问题，探索问题的规律性东西，要有一双敏锐的眼睛．请先观察、计算再填空．已知：如图，OM平分∠AOB，ON平分∠BOC．（1）当∠AOC=90°，∠BOC=70°时，∠MON=______；（2）当∠AOC=80°，∠BOC=60°时，∠MON=______；（3）当∠AOC=70°，∠BOC=50°时，∠MON=______；（4）猜想：不论∠AOC和∠BOC的度数是多少，∠MON的度数总等于______度数的一半．