-2/3,3/8,-4/15,5/24,-6/35
an=(-1)^n* (n+1)/[(n+1)²-1]
7,77,777,7777
an=(7/9)*(10^n -1)
-1,7,-13,19,-25,31
an=(-1)^n*(6n-5)
3/2,5/4,9/16,17/256
a(n)=(2^n +1)/[2^(2^(n-1))]