$\begin{aligned} & y=\cos \left(\sin x^2\right) \\ & \Rightarrow \frac{d y}{d x}=-\sin \left(\sin x^2\right) \cdot \cos x^2 \cdot 2 x \\ & \frac{d y}{d x}\left(a t x \sqrt{\frac{\pi}{2}}\right)=-\sin \left(\sin \frac{\pi}{2}\right) \cdot \cos \cdot 2 \times \sqrt{\frac{\pi}{2}} \\ & =-\sin (1) \times 0 \times 2 \times \sqrt{\frac{\pi}{2}}=0\end{aligned}$