18.[多选题] 设星形线{}x=acos^3t,y=asin^3t.t)dt.

18.[多选题] 设星形线$\left\{\begin{matrix}x=a\cos^{3}t,\\y=a\sin^{3}t.\end{matrix}\right.$下列求解正确的是（）.

A. 它所围图形的面积为$A=4\int_{\frac{\pi}{6}}^{\frac{\pi}{3}}a\sin^{3}t(-3\sin t\cdot\cos^{2}t)dt$.

B. 它的周长为$L=4\int_{\frac{\pi}{6}}^{\frac{\pi}{3}}3a\sin t\cdot\cos tdt$.

C. 它绕x轴旋转而成旋转体的体积为$V_{x}=3\pi a^{3}\int_{0}^{\frac{\pi}{2}}\sin^{7}t(1-\sin^{2}t)dt$.

D. 它绕x轴旋转而成旋转体的体积为$V_{x}=6\pi a^{3}\int_{0}^{\frac{\pi}{2}}\sin^{7}t(1-\sin^{2}t)dt$.