The answer is D just so you know
90-x because a complementary angle is =90 degrees.

so, let's first change the bounds from 1 to 9, so we'll be dropping the indices by 1, in order to do the variable "n".
however, let's use a variable hmmm say "k", where "k" is the value of "n" beginning at 1, thus
k = n - 1.
however, if k = n - 1, then
k + 1 = n. So let's use those fellows then,
4pi/2pi = x/360
use a proportion
2 radians equals 360 degrees
x = 720 degrees