3+ 1N(N+1)222=LIM 2021Å1ÃÅ1Ã =LIM 20211+21+22− 14+. . .+13−12−13+1N+...

2.3+ 1n(n+1)2=lim 2021Å1ãã =lim 20211+22− 14+. . .+13−12−13+1n+1n− 1Å= 20211+1=lim2021(n+1)2n =lim2021n