string(242) "get document error：https://zhidao.baidu.com/search?lm=0&rn=10&pn=0&fr=search&dyTabStr=null&word=%E6%A0%B9%E5%8F%B72x-1%E7%9A%84%E5%AF%BC%E6%95%B0%E6%98%AF%E5%A4%9A%E5%B0%91 => Failed to connect to 14.18.117.71 port 38069: Connection refused"
根号2x-1的导数是多少
