Help. ​

The tangent forms a right triangle.....AO  is the hypotenuse and AB is a leg....so.....by the Pythagorean Theorem......

r  = sqrt ( AO^2 - AB^2)  = sqrt ( 11.7^2 - 6^2 )   = sqrt ( 136.89 - 36) =

sqrt (  100.89) ≈  10.04  =  10 .0  (rounded)