Tính các giới hạn sau:
a) \(\mathop {\lim }\limits_{x \to 7} \frac{{\sqrt {x + 2} - 3}}\);
b) \(\mathop {\lim }\limits_{x \to 1} \frac{{{x^3} - 1}}{{{x^2} - 1}}\);
c) \(\mathop {\lim }\limits_{x \to 1} \frac{{{{\left( {1 - x} \right)}^2}}}\);
d) \(\mathop {\lim }\limits_{x \to - \infty } \frac{{\sqrt {4{x^2} + 1} }}\).