A Beginner's Guide to MathML
For displaying a square-root MathML provides the <msqrt> tag:
For more general roots there is the <mroot> tag. Unlike the <msqrt>, which can contain any number of child tags, the <mroot> must contain exactly two. Again the order is important: the first element is the expression being rooted, and the second is the power of the root.
Below is an example of a cubic root in which, to satisfy the two child rule , the expression being acted upon is grouped within an <mrow> tag: