To calculate the average rate of change, we need to look at the rate of change (AKA slope) of the function over the end points!
The formula for the slope of a line, f(x), over the interval [a,b] is f(b)-f(a)/(b-a)
So in this case, our function is 1/x and our interval is [8,9]
Therefore, the average rate of change of the function over the interval is
f(9)-f(8)/(9-8) =
(1/9-1/8)/(1)=
1/9-1/8 =-1/72