What is the diagonal of a regular pentagon whose radius is 20 cm?
Diagonal^2 = 20'2 + 20^2 -2 ( 20 *20) * cos (144°)
Diagonal^2 = 800 - 800 *(-1 - sqrt 5) / 4
Diagonal = sqrt [ 800 + 200 ( 1 + sqrt 5) ] = 10 sqrt [ 10 + 2sqrt 5 ] = 10 [ 2 (5 + sqrt 5)] ≈ 38.04
