a) Với m = 1 thì \(\mathop {\lim }\limits_{x \to - 1} g\left( x \right) = \mathop {\lim }\limits_{x \to - 1} \frac{{3x + 1}}{{ - x + 1}} = - 1\).

b) Với m = 1 thì \(\mathop {\lim }\limits_{x \to {1^ - }} g\left( x \right) = \mathop {\lim }\limits_{x \to {1^ - }} \frac{{3x + 1}}{{ - x + 1}} = + \infty \)

vì \(\mathop {\lim }\limits_{x \to {1^ - }} \left( {3x + 1} \right) = 4;\mathop {\lim }\limits_{x \to {1^ - }} \left( { - x + 1} \right) = 0\) và \( - x + 1 > 0\)khi x → 1−.

c) Với m = 1 thì \(\mathop {\lim }\limits_{x \to {1^ + }} g\left( x \right) = \mathop {\lim }\limits_{x \to {1^ + }} \frac{{3x + 1}}{{ - x + 1}} = - \infty \)

Vì \(\mathop {\lim }\limits_{x \to {1^ + }} \left( {3x + 1} \right) = 4;\mathop {\lim }\limits_{x \to {1^ + }} \left( { - x + 1} \right) = 0\) và \( - x + 1 < 0\)khi x → 1+.

d) \(\mathop {\lim }\limits_{x \to 3} f\left( x \right) = \mathop {\lim }\limits_{x \to 3} \frac{{\sqrt {5x + 1} - 4}}{{27 - {x^3}}}\)\( = \mathop {\lim }\limits_{x \to 3} \frac{{\left( {5x + 1 - 16} \right)}}{{\left( {27 - {x^3}} \right)\left( {\sqrt {5x + 1} + 4} \right)}}\)

\( = \mathop {\lim }\limits_{x \to 3} \frac{{5\left( {x - 3} \right)}}{{\left( {3 - x} \right)\left( {9 + 3x + {x^2}} \right)\left( {\sqrt {5x + 1} + 4} \right)}}\)\( = \mathop {\lim }\limits_{x \to 3} \frac{{ - 5}}{{\left( {9 + 3x + {x^2}} \right)\left( {\sqrt {5x + 1} + 4} \right)}}\)\( = \frac{{ - 5}}{{\left( {9 + 3.3 + {3^2}} \right)\left( {\sqrt {5.3 + 1} + 4} \right)}} = - \frac{5}{{216}}\) Þ \(a = - 5\).

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

Tìm m để A = −2025 với A = lim x → 1 x − m x + 2 .