If $y=f(u)$ and $u=g(x)$, where $f$ and $g$ are twice differentiable functions, it can be shown that
$$\displaystyle \frac{d^2y}{dx^2}= \frac{d^2y}{du^2}\left(\frac{du}{dx}\right)^2+\frac{dy}{du}\frac{d^2u}{dx^2}$$ .
Use this formula to compute $y''$ for $y=\sin(x^2)$.

$y'' =$