[m]\left\{\begin{matrix}
x`_{t}=(cos^3t)`=3cos^2t\cdot (cost)`=3cos^2t\cdot (-sint)\\ y`_{t}=(sin^3t)`=3sin^2t\cdot (sint)`=3sin^2t\cdot cost\end{matrix}\right.[/m]
[m]y`_{x}=\frac{3sin^2t\cdot cost}{3cos^2t\cdot (-sint)}=-tgt[/m]
[m]y``_{xx}=\frac{(y`_{x})`_{t}}{x`_{t}}[/m]
[m](y`_{x})`_{t}=(-tgt)`=-\frac{1}{cos^2t}[/m]
[m]y``_{xx}=\frac{(-\frac{1}{cos^2t})}{3cos^2t\cdot (-sint)}=\frac{1}{cos^4t\cdot sint}[/m]