题目

    One day a few years ago a very funny thing happened to a neighbour of mine. He is a teacher at one of London’s big medical schools, He had finished his teaching for the summer term and was at the airport on his way to Russia to give a lecture.     He had put a few clothes and his lecture notes in his shoulder bag, but he had put Rupert, the skeleton (人体骨骼) to be used in his lecture, in a large brown suitcase (箱子). At the airport desk, he suddenly thought that he had forgotten to buy a newspaper. He left his suitcase near the desk and went over to the shop.     When h若二次函数y=ax2+c，当x取x1，x2（x1≠x2）时，函数值相等，则当x取x1+x2时，函数值为________．