string(267) "get document error：https://zhidao.baidu.com/search?lm=0&rn=10&pn=0&fr=search&dyTabStr=null&word=%E6%A0%87%E5%87%86%E5%B7%AE%E4%B8%8E%E6%96%B9%E5%B7%AE%E5%85%AC%E5%BC%8F%E6%98%AF%E4%BB%80%E4%B9%88 => Failed to connect to 101.89.117.144 port 38208: Connection refused"
标准差与方差公式是什么
