Thomas' Calculus 13th Edition

$2 \pi a^3 \delta$
Given: $x^2+y^2=a^2$ and $I_z=\int_C (x^2+y^2) \delta ds=\int_C (a^2) \delta ds$ Thus, $I_z=a^2 \delta \int_C ds$ The circumference of the curve is given as: $\int_C ds =2 \pi a$; So, $I_z=2 \pi a^3 \delta$