I'm trying to understand the notation that the book uses.
The book says
$(1)$ $y=a\cdot \sin x$ and then the derivate of $(1)$ is $(2)$ $\frac{d^2y}{dx^2}=-a \cdot \sin x$
I don't get what to do when derivating to get $(2)$, and what exactly $\frac{d^2y}{dx^2}$ means.
The first derivative of $y=a \cdot \sin x$ is:
$$\frac{dy}{dx}=a \cdot \cos x$$
The second derivative of $y=a \cdot \sin x$,which is also the first derivative of $\frac{dy}{dx}=a \cdot \cos x$,is:
$$\frac{d^2y}{dx^2}=-a \cdot \sin x$$