The problem and solutions is here Isn't the shear stress for both the inner and outer negative? When i take the derivative i get negative. The derivative is basically ln ( 1/x) since the rest are just constants. Edit: I meant that the expression looks similar to ln (1/x). Also I am clueless to how they got the values for that the velocity destribution graph. They picked a/b = 0.8 and i believe the # has to be less than 1 or the Vz equation is undefined.