Chapter 10 - Section 10.6 - Radical Equations and Problem Solving - Exercise Set - Page 732: 71

12.97 feet

$P=2\pi\sqrt {I/32}$ $4=2\pi\sqrt {I/32}$ $4/2\pi=2\pi\sqrt {I/32}//2\pi$ $4/2\pi=\sqrt {I/32}$ $(2/\pi)^2=(\sqrt {I/32})^2$ $4/(\pi^2)=I/32$ $32*4/(\pi^2)=32*I/32$ $128/(\pi^2)=I$ $128/9.8696=I$ $12.969=I$