题目

    There once was a master who went to India. In those times, we didn't have the communications or airplanes or many kinds of transportation that we do now. So the master went to India on foot. He had never been to India before; perhaps he came from Persia. And when he got there, he saw a lot of fruit. In India they have plenty of fruit to sell, but much of it is expensive because they can't grow much due to the water situation. So he saw one basket, a big basket of some very red, long fruit. And it was the cheapest in the shop, not expensive at all.     So he went up and asked如果a2=-a，那么a的取值范围是______．