The recursive formula is of the form
a1 = first term
an = in terms of the previous term
The first term is 1
a1 =1
Looking at the function the second term is found by adding 2 to the first term
a2 = a1+2
The third term is found by adding 3 to the second term
a3 = a2+3
The fourth term is found by adding 4 to the 3rd term
a4 = a3+4
The an part of the recursive formula can be written as
an = a(n-1) + n
Answer: