First of all, you have to understand
is a square-root function.
Square-root functions are continuous across their entire domain, and their domain is all real x-values for which the expression within the square-root is non-negative.
In other words, for any square-root function
and any input
in the domain of
(except for its endpoint), we know that this equality holds:
Let's take
as an example.
The domain of
is all real numbers such that
. Since
is the endpoint of the domain, the two-sided limit at that point doesn't exist (you can't approach
from the left).
However, continuity at an endpoint only demands that the one-sided limit is equal to the function's value:
In conclusion, the equality
holds for any square-root function
and any real number
in the domain of
except for its endpoint, where the two-sided limit should be replaced with a one-sided limit.
The input
, is within the domain of
.
Therefore, in order to find
we can simply evaluate
at
.
