Help Remembering A formula

Question

I'm trying to understand how to get to calculate this formula: (n = 9). What does the "n" represent?

Answer 1

"n" often is used to mean a natural number, an integer greater than zero

such as in sequences

an arithmetic sequence has terms a1, a2, a3, ....an

for example

a1 = 3

a2 =7

a3 =11

...

an = a1 + (n-1)d where d = the common difference = 4

an=3 + (n-1)4 = 3-4+ 4n = 4n-1

an= 4n-1

a3 = 4(3)-1 = 11

a9 = 4(9)-1 = 35