Tìm các giới hạn sau:

a.\(\lim ( - {n^3} + n - 3) = \lim {n^3}( - 1 + \frac{1}{{{n^2}}} - \frac{3}{{{n^3}}}) = - \infty \)

b. \(\mathop {\lim }\limits_{x \to 2} \frac{{{x^2} - 5x + 6}}{{x - 2}} = \mathop {\lim }\limits_{x \to 2} \frac{{\left( {x - 2} \right)\left( {x - 3} \right)}}{{x - 2}}\)\( = \mathop {\lim }\limits_{x \to 2} \left( {x - 3} \right) = - 1\).