Giới hạn \[\lim \left( {\sqrt {{n^2} - n + 1} - \sqrt {{n^2} + 1} } \right)\] bằng?

\[\begin{array}{*{20}{l}}{\lim (\sqrt {{n^2} - n + 1} - \sqrt {{n^2} + 1} )}\\{ = \lim \frac{{(\sqrt {{n^2} - n + 1} - \sqrt {{n^2} + 1} )(\sqrt {{n^2} - n + 1} + \sqrt {{n^2} + 1} )}}{{\sqrt {{n^2} - n + 1} + \sqrt {{n^2} + 1} }}}\\{ = \lim \frac{{{n^2} - n + 1 - {n^2} - 1}}{{\sqrt {{n^2} - n + 1} + \sqrt {{n^2} + 1} }}}\\{ = \lim \frac{{ - n}}{{\sqrt {{n^2} - n + 1} + \sqrt {{n^2} + 1} }}}\\{ = \lim \frac{{ - 1}}{{\sqrt {1 - \frac{1}{n} + \frac{1}{{{n^2}}}} + \sqrt {1 + \frac{1}{{{n^2}}}} }}}\\{ = - \frac{1}{2}}\end{array}\]

Đáp án cần chọn là: B