I was reading a solution to calculating some integral. one of the steps is:
$$\int xe^{-x^2}\sin(ax) dx = -\frac{1}{2}\int \sin(ax) de^{-x^2}$$
I am not familliar with the notation $de^{-x^2}$ because I normally use $du$ and write $u=....$. But in this case I understand it.
Now the next step I dont get:
$$\int \sin(ax) de^{-x^2}=-\int e^{-x^2} d\sin(ax)$$
What happens here?? I can't get the substitution $u=, du=...dx$ steps clear to do it myself