a)
\mathop {\lim }\limits_{x \to ^{+}_{-}\infty }(x+\sqrt[3]{3x^{2}-x^{3}})
b)\mathop {\lim }\limits_{x \to ^{+}_{-}\infty }\frac{x^{2}+\sqrt[3]{1-x^{6}}}{\sqrt{x^{4}+1}-x^{2}}
c)\mathop {\lim }\limits_{x \to a}\frac{x-a}{x^{n}-a^{n}}
d)\mathop {\lim }\limits_{x \to ^{+}_{-}\infty }\sqrt{x+\sqrt{x+\sqrt{x}}}-\sqrt{x}