Tính các giới hạn:
a) $\mathop {\lim }\dfrac{2n^2+n+1}{3n^2+1}$ b) $\mathop {\lim }\dfrac{n^3+n\sqrt{n}-3}{n^3-2n+5}$
c) $\mathop {\lim }\dfrac{3n^3\left(2n+1\right)^{10}}{\left(2n^2-1\right)^8\left(2n-3\right)^5}$ d) $\mathop {\lim }\dfrac{\sqrt{n^2+n}+n}{\sqrt{4n^2+1}+n-1}$