a/ \(=\lim\limits_{x\rightarrow-\infty}\dfrac{\dfrac{3x^4}{x^4}-\dfrac{2x^5}{x^4}}{\dfrac{5x^4}{x^4}+\dfrac{x}{x^4}+\dfrac{4}{x^4}}=+\infty\)
b/ \(=\lim\limits_{x\rightarrow+\infty}\dfrac{\dfrac{x}{x}-\dfrac{1}{x}}{\sqrt{\dfrac{x^2}{x^2}-\dfrac{1}{x^2}}}=1\)