21: After substituting in the limit point you should find that
the expression is of the type
∞ − ∞.
The leading
term under the root is n3, so when the root is taken into
account, the first term behaves like n when n goes to infinity,
that is, it is of the same order as the second term
"-n".
Intuitive calculations
would make this leading term disappear,
so factoring out would not help.
Instead the best bet seems to be getting rid of the cubic root
algebraically, see the box
"difference of roots".
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