string(230) "get document error：https://zhidao.baidu.com/search?lm=0&rn=10&pn=0&fr=search&dyTabStr=null&word=%E5%AF%BC%E6%95%B0%E7%9A%84%E5%AE%9A%E4%B9%89%E7%BB%8F%E5%85%B8%E4%BE%8B%E9%A2%98 => Received HTTP code 403 from proxy after CONNECT"
导数的定义经典例题
