The Navier-Stokes equations are solved exactly, assuming the Navier slip boundary condition, for flow
between coaxial circular cylinders (with possibly different slip lengths at the inner and outer walls), and flow in an
equilateral triangular pipe. The velocity fields and total flow rates are found analytically. A relatively small slip length can
lead to a large increase in the total rate of fluid flow: in the triangular pipe case the flow doubles when the slip length is
only about 7.5% of the triangle side.