lim
sin[π(n^2+1)^1/2]=lim
{sin[π(n^2+1)^1/2]-sin
nπ}
=lim
cos(π/2)((n^2+1)^1/2+n)
sin
(π/2)[(n^2+1)^1/2-n)]
=lim
cos(π/2)((n^2+1)^1/2+n)
sin
(π/2){1/[(n^2+1)^1/2+n)]}.
cos(π/2)((n^2+1)^1/2+n)有界,lim
sin
(π/2){1/[(n^2+1)^1/2+n)]}=0
有界与无穷小之积仍是无穷小。所以
.lim
sin[π(n^2+1)^1/2]=0