a(4) =13
a(1) = 1
a(2)=5
a(3)=9
a(4)=13
a(5)=17
So, the arithmetic formula is: a(n) = a(n-1) + d
If we look at the formula, the numbers change by +1, which is our d
We can test our formula by look for 2,
a(2)= a(2-1)+4
a(1)+4
1+4
a(2)=5
Even though the sequence tells us, we can test our formula anyhow
a(4)= a(4-1)+4
a(3)+4
9+4
a(4)=13