Solution
For this case we know the diameter:
D= 9 ft
Then the radius is:
r= 9/2 ft
Then we can use the following formula
[tex]\text{mAB}=2\pi r(\frac{\theta}{360})[/tex]Then the measure would be:
[tex]\text{mAB}=2\pi\cdot\frac{9}{2}(\frac{80}{360})=2\pi[/tex]