To do so we need to analyze how the consecutive terms are related. Let's find the difference between each pair of consecutive terms.
ccccc
a_2-a_1&=& 4-1 &=& 3 [1.2em]
a_3-a_2&=& 7-4 &=& 3 [1.2em]
a_4-a_3&=& 11-7&=& 3 [1.2em]
... & & ... & & ...
a_n-a_(n-1) = 3
We can see above that the difference between the consecutive terms equals 3. Therefore, to obtain the value of the term in the n^(th) position, we need to add 3 to the previous term. With this information and knowing that the first term equals 1, we can write the recursive formula.
a_1=1 and a_n=a_(n-1) +3
