Khẳng định nào sau đây sai?

a) Khi \(m = - 1\) thì \(\mathop {\lim }\limits_{x \to {2^ - }} f\left( x \right) = \mathop {\lim }\limits_{x \to {2^ - }} \left( {{x^2} - 1 - 2} \right) = \mathop {\lim }\limits_{x \to {2^ - }} \left( {{x^2} - 3} \right) = 1\).

b) \(\mathop {\lim }\limits_{x \to 3} f\left( x \right) = \mathop {\lim }\limits_{x \to 3} \sqrt {x + 7} = \sqrt {10} \).

c) \(\mathop {\lim }\limits_{x \to {2^ + }} f\left( x \right) = \mathop {\lim }\limits_{x \to {2^ + }} \sqrt {x + 7} = 3\); \(\mathop {\lim }\limits_{x \to {2^ - }} f\left( x \right) = \mathop {\lim }\limits_{x \to {2^ - }} \left( {{x^2} - 1 + 2m} \right) = 3 + 2m\); \(f\left( 2 \right) = 3\).

Để tồn tại \(\mathop {\lim }\limits_{x \to 2} f\left( x \right)\) thì \(3 + 2m = 3 \Leftrightarrow m = 0\).

d) \(\mathop {\lim }\limits_{x \to {2^ + }} f\left( x \right) = \mathop {\lim }\limits_{x \to {2^ + }} \sqrt {x + 7} = 3\).

Đáp án: a) Đúng;    b) Sai;   c) Sai;   d) Đúng.

\(M = \lim \left( {\sqrt {9{n^2} - 3n + 7} - 3n} \right)\)\( = \lim \frac{{ - 3n + 7}}{{\sqrt {9{n^2} - 3n + 7} + 3n}}\)\( = \lim \frac{{ - 3 + \frac{7}{n}}}{{\sqrt {9 - \frac{3}{n} + \frac{7}{{{n^2}}}} + 3}} = - \frac{1}{2}\).

Suy ra \(a = 1;b = 2\). Do đó \(a + b = 3\).

Trả lời: 3.