\(=\frac{1 I _{ b } I _{ t } \omega_{ i }^2}{2\left( I _{ t }+ I _{ b }\right)} \)
\( \therefore \Delta E =\frac{1 I _{ b } I _{ t }}{2\left( I _{ t }+ I _{ b }\right)} \omega_{ i }^2\)
Hence, the answer is \(\frac{1}{2\left( I _{ t }+ I _{ b }\right)} \omega_{ i }^2\).