lim(n-> ∞)[(1+2+...+n)/(n+2)- n/2]
=lim(n-> ∞)[n(n+1)/[2(n+2)]- n/2]
=lim(n-> ∞)[n(n+1)-n(n+2) ]/[2(n+2)]
=lim(n-> ∞)-n/[2(n+2)]
=lim(n-> ∞)-1/[2(1+2/n)]
=-1/2
lim(n-> ∞)[(1+2+...+n)/(n+2)- n/2]
=lim(n-> ∞)[n(n+1)/[2(n+2)]- n/2]
=lim(n-> ∞)[n(n+1)-n(n+2) ]/[2(n+2)]
=lim(n-> ∞)-n/[2(n+2)]
=lim(n-> ∞)-1/[2(1+2/n)]
=-1/2