What subject is this if this is math I am good at it
I'd do 9x4=36 because the 9 is from your money and the 4 is from your books. Now since you want to find out how much 4 books cost you just multiply that by 9
For every
, we have

So, negletting the first term, each term of the sum is greater than 1, which implies

Which means that the series diverges.
add the 2 numbers and then divide by 2