Tìm các giới hạn sau:
a) $\mathop {\lim }\limits_{x \to 1}\left(x+1\right)\sqrt{\dfrac{2x+1}{x^3+x+2}}$
b) $\mathop {\lim }\limits_{x \to 2}\dfrac{\sqrt[3]{8x+11}-\sqrt{x+7}}{x^2-3x+2}$
c) $\mathop {\lim }\limits_{x \to+\infty}\left(1-2x\right)\sqrt{\dfrac{3x-11}{x^3+1}}$