Cb is tangent to ⊙a at point c. find the radius. cb ⊥ ac by the radius-tangent theorem, so ∠c is a right angle. δabc is a right triangle, so apply the pythagorean theorem. use the steps and solve for the radius. r2 + 82 = (r + 5)2 r2 + 64 = r2 + 10r + 25 r =