Answer:
r = .5 units
Explanation:
8.7 R is an improper FRACTION of the 2 pi R of the entire circle ....set up a ratio:
8.7 / (2 pi) = 3.9 / x where x is the entire circumference measurement
x = 3.9 / (8.7 / (2 pi) ) = 2.82 units
To find the radius circumf = 2 pi r
2.82 = 2 pi r shows r = .5 units