The radii of two concentric circles centre O are 26 cm and 16 cm. Chord AB of the large circle is tangent to the smaller circle at C and AD is diameter. What is the length of CD?

For concentric circles with radii 26 cm (outer) and 16 cm (inner), chord AB of outer circle is tangent to inner circle at C. By tangent property, OC is perpendicular to AB. Since OC = 16 cm and OA = 26 cm, using right triangle OCA: AC = √(OA² - OC²) = √(676 - 256) = √420 = 2√105. By symmetry, C is midpoint of AB, so AB = 2AC = 4√105. AD is diameter = 52 cm. Using intersecting chords theorem or properties of triangles, CD = 38 cm.