Answer:
r = 3
Explanation:
Pythagoras' theorem states that for all right-angled triangles, 'The square on the hypotenuse is equal to the sum of the squares on the other two sides.
AO^2 = AB^2 + BO^2
(2 + r)^2 = 4^2 + r^2
r^2 + 4r + 4 = 16 + r^2
4r = 12
r = 12/4
r = 3