${u_n} = \sqrt {{{{1^2} + {2^2} + ... + {n^2}} \over {\left( {{n^2} + n} \right)\left( {n + 2} \right)}}}$          b)${u_n} = {{{1^3} + {2^3} + ... + {n^3}} \over {\sqrt {{n^7} + 3{n^4} + 1} }}$

${u_n} = \root 3 \of {n - 2{n^3}}$    

${u_n} = {2^n} - {4.3^{n + 1}}$

${u_n} = 100n - {2.5^n}$      

 ${u_n} = {{{3^n} - {4^{n + 1}}} \over {{2^{2n}} + {{10.3}^n} + 7}}.$