题目

In 1945,there was a young boy of 14 in a concentration camp （集中营）. He was tall, thin but had a bright smile. Every day, a young girl came by the other side of the fence. She noticed the boy and asked him if he spoke Polish, and he said yes. She then reached in her pocket and gave him her apple. He thanked her and she went on her way. The next day, she came by again, bringing with her another apple which she gave him. Each day, she walked by the outside of the fence, hoping to see him, and when she did, she happily handed him an apple in exchange for conversation.  On day he设实数x，y满足约束条件，若目标函数z=+（a＞0，b＞0）的最大值为9，则d=的最小值为    ．