The shaded region represents 1/3 of the circle. If the radius of the circle is 9 mm, what is the perimeter of the sector in terms of pi?​

answer: c is correct, a regular pentagon

step-by-step explanation:

points a and b are equidistant from the desired line, so the line you find by using them will go through point r.

using points other than a and b will give the desired line only if they are equidistant from r. if they are randomly located, the perpendicular line you create will not necessarily go through point r, hence will not be a solution to the problem.