Answer:
r = 15
Step-by-step explanation:
Using Pythagoras' identity in the right triangle OAB
with OM and OB the radii r of the circle
OA² = OB² + AB²
(r + 10)² = r² + 20² ← expand left side using FOIL
r² + 20r + 100 = r² + 400 ( subtract r² from both sides )
20r + 100 = 400 ( subtract 100 from both sides )
20r = 300 ( divide both sides by 20 )
r = 15
