Bisection Method using Prime 3

Hi

I'm using prime 3 and I want to write a bisection method code but I'm getting an error as follow :

1/ I can't write (i+1) as a subscript for the variable name.

For this problem, I think I solved it by defining a new variable j as (i+1), but I'm not sure if this is correct or not ?

2/ I get an error telling me that the (ai) variable is undefined !!

Please help and see the attachment