等价无穷小的替代http://wenku.baidu.com/link?url=kDXc56ln8GzVavRkb_MU-ZzwvYGUxbQwAUdp0FcdAZt9VQgJg_b3sU9w5NeARzPRVbtXIMELMSk_Eg6TK3PjWmlUll66gRXy6_6gyYA2s03你可以去看一下
