Xét sự hội tụ
$A=\sum_{2}^{\infty }\frac{\left ( lnn \right )^{7}}{(n+1)^{1/2}}$
$B=\sum_{1}^{\infty }\left ( \frac{n+7}{n+8} \right )^{n}$
$C=\sum_{1}^{\infty }\frac{n^{7}-1}{n^{x}}$
$D=\sum_{1}^{\infty }\frac{\left ( -1 \right )^{n}.n^{7}}{n^{5}+2}$
$A=\sum_{2}^{\infty }\frac{\left ( \ln n \right )^{7}}{(n+1)^{1/2}}$ phân kỳ vì $\frac{\left ( \lnn \right )^{7}}{(n+1)^{1/2}} \ge \frac{1}{2\sqrt{n}} \forall n\ge 3.$
$B=\sum_{1}^{\infty }\left ( \frac{n+7}{n+8} \right )^{n}$ phân kỳ vì $ \lim a_n=e.$
$C=\sum_{1}^{\infty }\frac{n^{7}-1}{n^{x}}$
($x$ ????)
$D=\sum_{1}^{\infty }\frac{\left ( -1 \right )^{n}.n^{7}}{n^{5}+2}$ phân kỳ vì $ \lim |a_n|= \infty.$