∵l=
nπr
180 =
8nπ
20
9 π
∴n=50°即∠AOC=50°
∵AC为圆的切线,∴∠ACO=90°
∴AO=
CO
cosO ≈12.45(cm)
∴AB=AO-OB=12.45-8=4.45(cm).