A passenger（旅客）told an air hostess that he needed a cup of water to take his medicine when the plane just took off. She told him that she would bring him the water in ten minutes.
The air hostess was kept so busy that she forgot to give him the water. As a result, the passenger was held up(延误) to take his medicine. Thirty minutes later, when the passenger's ring for service sounded, she hurried over to him with a cup of water, but he refused.
In the following hours on the plane, each time the air hostess passed by the passenger, she would ask him with a smile whether he needed help or not. But the passenger never answered a word.
When he was going to get off the plane, the passenger asked the air hostess to hand him the passengers' booklet（小册子）. She was very sad. She knew that he would write down sharp words, but with a smile she handed it to him.
Off the plane, she opened the booklet, and smiled, for the passenger put it. “On the flight, you asked me whether I needed help or not for twelve times in all. How can I refuse your twelve sincere smiles?”That's right! Who can refuse twelve sincere smiles from a person?
≥a恒成立，则实数a的取值范围是( )