For each sequence given below, find a closed formula for an, the nth term of the sequence (assume the first terms here are always ao) by relating it to another sequence for which you already kno the formula.
a. 1, 3, 9, 19, 33, 51, ...
an=
b. 0, 5, 15, 30,50, 75, ...
an =
c. 3, 4, 6, 10, 18, 34,..
an =

d. 0, 2, 7, 15, 26, 40, ...
An=