Tìm các giới hạn:
a) \(\mathop {\lim }\limits_{x \to 3}\frac{x+3}{x^{2}+9}\)
b) \(\mathop {\lim }\limits_{x \to 4}\frac{\sqrt{1+2x}-1}{2x}\)
c) \(\mathop {\lim }\limits_{x \to 6}\frac{4x}{\sqrt{9+x}-3}\)
d) \(\mathop {\lim }\limits_{x \to 2}\frac{\sqrt[3]{4x}-2}{x+2}\).
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