how do you put the radical key?

If you are writing in the input box you can enter

sqrt(5)     which will give you     $${\sqrt{{\mathtt{5}}}}$$              Note that this is the same as  5^(1/2)          $${{\mathtt{5}}}^{\left({\frac{{\mathtt{1}}}{{\mathtt{2}}}}\right)}$$

OR

sqrt(5,3)   which will give you     $${\sqrt[{{\mathtt{{\mathtt{3}}}}}]{{\mathtt{5}}}}$$           Note that this is the same as  5^(1/3)          $${{\mathtt{5}}}^{\left({\frac{{\mathtt{1}}}{{\mathtt{3}}}}\right)}$$