\(\displaystyle{\int_0^{2\pi a}\pi y^2\ dx}\)
\(\displaystyle{=\int_0^{2\pi}a^3(1-\cos t)^3\ dt}\)
\(=5\pi a^3\)
\(\displaystyle{\int_0^{2\pi}\cos^3 t\ dt=0}\) である説明
\(\displaystyle{\int_0^{2\pi}\cos^3 t\ dt}\)
\(\displaystyle{=\int_0^{\pi}\cos^3 t\ dt}\)
\(\displaystyle{+\int_{\pi}^{2\pi}\cos^3 t\ dt}\)
\(\displaystyle{=\int_0^{\pi}\cos^3 t\ dt}\)
\(\displaystyle{+\int_{0}^{\pi}\cos^3 (\pi+t)\ dt}\)
\(\displaystyle{=\int_0^{\pi}\cos^3 t\ dt}\)
\(\displaystyle{+\int_{0}^{\pi}(-\cos t)^3\ dt=0}\)