Write the recursive formula for 5, 15, 30, 50, . . .

A recursive sequence is a sequence in which terms are defined using one or more previous terms that are given.
 If you know the term n of an arithmetic sequence and you know the common difference, d, you can find the term (n + 1) by tossing the recursive formula to n + 1 = a n + d

 We have then:
 5, 15, 30, 50
 an = (5/2) * (n) * (n + 1)
 
 the recursive formula for 5, 15, 30, 50, is: 
 an = (5/2) * (n) * (n + 1)

7for nonmembers i think