Since we know that ##Q = \text{Total charge uniformly distributed in the disc}## and the area of the disc is just ##\pi((b_1-b_0)^2)##.
We can define ##dQ_{\tiny{|}}## as the charge of an infinitesimally small point on the disk, and ##Q_{\tiny{|}}## as the chage of an infinitesimally small cross...