Tính 2a + b2.

D

\[\mathop {\lim }\limits_{x \to 4} \frac{{\sqrt {3x + 4}  - 4}}{{x - 4}} = \mathop {\lim }\limits_{x \to 4} \frac{{\left( {3x - 12} \right)}}{{\left( {x - 4} \right)\left( {\sqrt {3x + 4}  + 4} \right)}}\]\[ = \mathop {\lim }\limits_{x \to 4} \frac{3}{{\sqrt {3x + 4}  + 4}} = \frac{3}{8}\].

Suy ra a = 3; b = 8. Do đó 2a + b2 = 6 + 64 = 70.