自古以来，我国就是统一的多民族国家，中华民族和各民族的关系，是一个大家庭和家庭成员的关系。在长期的历史发展中，各民族形成你中有我、我中有你，谁也离不开谁的多元一体格局的原因在于（   ）

①祖国的统一大业是各族人民共同完成的②祖国的辽阔疆域和多种经济是各族人民共同开拓和发展的③祖国的灿烂文化是各族人民共同创造的④祖国的新型民族关系是各族人民共同的血缘联系决定的

若a＜b＜0，则（a+b）（a﹣b）____ 0．

    When Mr. David retired(退休), he bought a small house in a village near the sea. He liked it and hoped to live a quiet life in it. But to his surprise, many visitors came to see his house in summer holidays, for it was the most interesting building in the village. From morning to night, there were visitors outside the house. They kept looking into the rooms through the windows and many of them even went into the house. He decided to drive the visitors away. So he put a notice on the window. The notice said，“If you want to satisfy your curiosity(好奇心), come in and look around. Price: twenty dollars.”Mr. David was sure that the visitors would stop coming, but he was wrong. More and more visitors came and Mr. David had to spend every day showing them around his house. “I came here to retire, not to work as a guide(导游)”, he said angrily. In the end, he sold the house and moved away.

为了炼出某种特定用途的钢材，炼钢时需要加入一定量的某种化学元素，已知每炼1吨钢需要加入这种化学元素的量在1000，2000内（单位：g），采用0.618法确定最佳加入量，设第1，2，3个试点的加入最分别为x₁，x₂2，x₃（x₁＞x₂），若第1个试点比第2个试点好，则第3个试点的加入量x₃=____