Let $A,$ $B,$ and $C$ be points on a circle of radius $18.$ If $\angle ACB = 70^\circ,$ what is the circumference of the minor arc ${AB}$? Express your answer in terms of $\pi.$
Because the angle $\angle ACB$ is $70^\circ$, the arc $AB$ is $140^\circ$. The circumference of the whole circle is $2\pi(18)=36\pi$. Because the minor arc $AB$ is $140^\circ$, the length of the minor arc $AB$ is $\frac{140}{360}(36\pi)=\frac{7}{18}(36\pi)=14\pi$.

