## Thomas' Calculus 13th Edition

The arc should be $8.4in$ long.
$\theta = 80^\circ=\frac{80}{180}\pi=\frac{4}{9}\pi$ $D=12in$ $r=\frac{D}{2}=6in$ $s=r\theta = \frac{4}{9}\pi*6in\approx 8.4in$