Total moment of shear stress about the center line of a cylinder

[eterna]

http://postimg.org/image/onxjqc26l/

I have found the shear stress of the inner cylinder to be 405 Pa like in the solution
but I am having trouble visualizing the total moment of this stress about the center line.
I was wondering if someone could explain where that d(theta) in the moment expression comes from, possibly with a diagramthanks

 

[Chestermiller]

The d(theta) comes in because you have to integrate around the circumference of the cylinder. The force on a differential area of the cylinder is $dF=\tau (L r_id\theta)$.