string(249) "get document error：https://zhidao.baidu.com/search?lm=0&rn=10&pn=0&fr=search&dyTabStr=null&word=%E4%BB%80%E4%B9%88%E7%9A%84%E5%AF%BC%E6%95%B0%E6%98%AF2%E7%9A%84x%E6%AC%A1%E6%96%B9 => Failed to connect to 61.171.74.35 port 38041: Connection refused"
什么的导数是2的x次方
