The radii of two concentric circles are 13 cm and 8 cm. AB is a diameter of the bigger circle and BD is a tangent to the smaller circle touching it at D and the bigger circle at E. Point A is joined to D. The length of AD is

• let o be the center so OAD be a rightangled triangle;
  OD=8;OA=13;AD=sqrt(13^2+8^2)=15.26
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