Exercise 19 Page 867

Recall that the measure of each interior angle of a regular $n\text{-gon}$ is $\frac{(n-2)180}{n}.$ To find the measure of an interior angle of an octagon, we have to substitute $8$ for $n$ in this expression. Because there are $8$ sides, there are $8$ angles. We can multiply this angle measurement by $8$ to find the sum of the angles. Therefore, the correct answer is D.