When dealing with sums or differences of square roots, we sometimes need
to rationalize the expression. The key fact is that
(√A+√B)(√A−√B)=A−B,
so
√A−√B=A−B√A+√B,
no matter what A and B are (as long as they are non-negative). For
example, instead
of working with √x+1−√x, it is sometimes easier to work with
(√x+1−√x)⋅√x+1+√x√x+1+√x=x+1−x√x+1+√x=1√x+1+√x.
This trick works both with square roots in the numerator and with square
roots in the denominator:
1√A−√B=√A+√BA−B;
and also in cases like
A±√B, or 1A±√B.
The following video shows how to use this trick to get limits.