Question

The radii of two concentric circle are 13cm and 8cm.AB is diameter ofthe bigger circle. BD is the ttangent to the smaller circle touching it at D. Find the length AD

Answer 1

see diagram and solution enclosed.

To know AD, we need to know AC & CD.  

To find them, we need BC and BD first.

Given   OA=OB = R ,   OD= r

Let the radii of circles be R & r.   R > r.     R= 13cm   r =  8cm.

Finally:   AD = √(R²+3 r²) = √(13²+3*8²) = 19 cm