题目

   Every year thousands of tourists visit Pompeii, they see the sights that Pompeii is famous for ---its stadium(运动场) and theatres, its shops and restaurants. The tourists do not, however, see Pompeii’s people. They do not see them because Pompeii has no people. No one has lived in Pompeii for almost 2,000 years. Once, Pompeii was a busy city of 22,000 people. It lay at the foot of Mount Vesuvius, a grass-covered volcano. Mount Vesuvius had not erupted for centuries, so the people of Pompeii felt safe. But they were not. In August of AD 79, Mount Vesuvius erupted. The entire t计算：[（2x+y）2-（2x+y）（2x-y）]÷2y-12y．