Tính tích phân \[I = \mathop \smallint \limits_0^a 4{x^2}\sqrt {{a^2} - {x^2}} dx\]

\[\frac{{ - \pi {a^4}}}{4}\]

\[\frac{{\pi {a^4}}}{4}\]

\[\frac{{\pi {a^3}}}{8}\]

\[\frac{{\pi {a^2}}}{4}\]