(3n^4 -2) < (3n^4)^(1/3)
(3n^4 -2)^(1/3) ∞) { 1/(3n^4)^(1/3)}/{1/(3n^4 -2)^(1/3) }
=lim(n->∞) { [1-2/(3n^4) }^(1/3)}
= 1 > 1/2
∴ 由极限的保序性 ,当n>N时:
{ 1/(3n^4)^(1/3)}/{1/(3n^4 -2)^(1/3) } > 1/2
1/2 *1/(3n^4 -2)^(1/3) < 1/(3n^4)^(1/3)
此时由 ∑1/(3n^4)^(1/3) 的收敛性,当然可推出1/2 *1/(3n^4 -2)^(1/3) 或 1/(3n^4 -2)^(1/3) 的收敛性.
由上可知,比较原则的核心即在于考察此无穷小量的阶或主要部分,而简化其值,所以单纯比较无穷小量大小并无必要.