As is known是
定语从句，as可以指代后面被修饰的主句。eg. As is known to all, Beijing is the capital of China.（众所周知，北京是中国的首都）
What is known是引导
宾语从句的，用what来指代。eg.What is known is that Mary is the best student in our class. （众所周知，玛丽是班上最优秀的学生）
It is known 是用来引导
主语从句的。当主语是一个较长的句子时为了避免“头重脚轻”，我们将其“甩”到后面，用it来指代。eg。It is known that
New York is the business center in the world. (众所周知，纽约是世界上的商业中心)
翻译成中文的话，这三个表达用法没什么差别
还有亲，你为什么要把这道题归类到数学呢？下次可要注意了哦。期待能为你解决困惑