Tính các giới hạn:
a) $\mathop {\lim }\dfrac{6+10+14+...+\left(4n+2\right)}{n^2+4}$ b) $\mathop {\lim }\left(2n-\sqrt{4n^2+n}\right)$
c) $\mathop {\lim }\left(\sqrt{n^2+4n}-n+2\right)$ d) $\mathop {\lim }\left(\sqrt{n^2+n+1}-\sqrt[3]{n^3+n^2}\right)$