Hàm số \[x = a.{\cos ^3}t,y = b.{\sin ^3}t,t \in (0,\frac{\pi }{2})\] có x'(t) là:

\[ - 3a{\sin ^2}t\sin t \ne 0,\forall t \in (0,\frac{\pi }{2})\]

\[ - {\cos ^2}t\sin t \ne 0,\forall t \in (0,\frac{\pi }{2})\]

\[ - 3a{\cos ^2}t\sin t \ne 0,\forall t \in (0,\frac{\pi }{2})\]

\[ - 3a{\cos ^2}t\sin t\sin t \ne 0,\forall t \in (0,\frac{\pi }{2})\]