an = an-1 +d
Sequence
1,4,7,........
4-1 =3
7-4 =3
d=3
a_n =a_(n-1) +3
check:
a_1 =1
a_2 = a_1 +3 = 1+3 =4
2_3 = a_2 + 3 = 4 + 3 = 7
