(a) Show that the polar radius of gyration $k_{o}$ of the annular area

shown is approximately equal to the mean radius $R_{m}=\left(R_{1}+R_{2}\right) / 2$ for small values of the thickness $t=R_{2}-R_{1} .(b)$ Determine the percentage error introduced by using $R_{m}$ in place of $k_{O}$ for the following values of $t / R_{m}: 1, \frac{1}{2},$ and $\frac{1}{10} .$