In a physics book it is said that we can reduce the following expression with only $Q$ and $r$ like $Qr$ or $\dfrac{dQ}{dr}$ or something like that ..... so that it gets vanished after closed integration:
$$\int r\dfrac{d^{2}Q}{dsds'}ds'$$
It should be possible with integration by parts or with some other techniques.
My try:
I tried with integration by parts to get: $$r\dfrac{dQ}{ds}-\int\dfrac{dr}{ds'}\dfrac{dQ}{ds}ds'$$
I can't get any further. Please help.
Edit:
It is a well known fact that any value of $Q$ is valid as far we are considering closed circuits because it gets cancelled out after closed integration. You can see it on Wikipedia$-$Ampere Force Law$−$Historical Background