In the above image we can see to find area of circle they consider the very small annulus with area of $\delta A=2\pi r \delta r$. and in order to to find $\int u^3dA$ and it plugged in equation of $u_r$ into integral.
My question is : We consider the $\delta A$ as area of annulus and find the integral with bound $r\in [0,R]$. which give the area of circle. I wonder what we call this small area $\delta A$ in math? Is this riemenn?