Ответ:

9 см

Объяснение:

sin(a)^2 + cos(a)^2 = 1

cos(a)^2 = 1 - sin(a)^2

(√11/6)^2 = 1-sin(a)^2

sin(a)^2 = 1 - 11/36

sin(a) = √25/36

sin(a) = 5/6

15/6 = 2r

2r = 15/6 * 6

2r = 6*3

r = 9