Question

find the recursive formula ... 3, -7, -17, -27,.......

Answer 1

the negative sign isnt changing back and forth, so its not a geometric sequence

the difference between each term looks to be constant at 10

a1= 3
a2=-7= a1-10
a3= -17 =a2-10

oh look a pattern
an= a(n-1) - 10

Answer 2

first you have to know that each number is by adding by 10 so to start your formula it will be f(n)=f(n-1)+10
N will be the first number(3)
so it would be f(3)=f(3-1)+10

I not full 100% if i explain it right but here's a link as well
https://learnzillion.com/lesson_plans/5940-create-a-recursive-formula