a) \( \lim{ \frac{ \sqrt{n^3+7} -n^2 +n }{n^2+6n-\sqrt[3]{n^4} } } = ? \)
b) \( \lim{ \frac{ \sqrt[3]{n^4-8n} + n^2+3n }{ \sqrt{9n^4+1}-\sqrt[3]{n^5+n^4}+n-n^2 } }= ? \)
c) \( \lim{ \sqrt{ \frac{ 4^{n+1}-5 }{ 2^{2n+1}+1 } } } = ? \)
d) \( \lim{ \sqrt[3]{ \frac{ 24n^5-12n^3+3n }{ 7n-n^2-3n^5 } } } = ? \)