用比值法
设Un=1/(n+3^n)
Un+1=1/[n+1+3^(n+1)]
lim n→∞ Un+1/Un
=lim 1/[n+1+3^(n+1)] / 1/(n+3^n)
=lim (n+3^n)/[n+1+3^(n+1)]
=lim [n/(3^n) +1]/[n/(3^n)+1/(3^n)+3]
=(0+1)/(0+0+3)
=1/3<1
所以该级数收敛。
追问这书上的解析(4)中前几项是什么意思
![](https://video.ask-data.xyz/img.php?b=https://iknow-pic.cdn.bcebos.com/3bf33a87e950352a8a2319005943fbf2b2118b73?x-bce-process=image%2Fresize%2Cm_lfit%2Cw_600%2Ch_800%2Climit_1%2Fquality%2Cq_85%2Fformat%2Cf_auto)
追答调和级数1/n=1/1+1/2+1/3……
而1/(n+3=1/4+1/5+……
少了前三项,实际上还是调和级数
麻烦给个采纳吧