Correct Answer!!!
We use the :
lim
n
®
¥
| a
n+1
|
| a
n
|
=
lim
n
®
¥
( n+4) !
3!( n+1)!3
n+1
3!n!3
n
( n+3) !
=
lim
n
®
¥
n+4
3( n+1)
=
1
3
<
1
Therefore the series (it has positive terms only).