The formula you wrote is wrong, d=(an-am)/(n-m), that is, tolerance = the difference between any two terms is greater than the difference between these two subscripts.

A7+a9=2a8= 10，a8=5，a4= 1。

So d=(a8-a4)/(8-4)= 1,

The general term an=a4+(n-4)d= 1+n-4=n-3,