Equation question from Reichmann X-C Soaring

I am working with a fourth printing (1988) and the equation presented
for Optimum Average Cruise Speed does not make sense. It is equation
VI in the book, but it gives a very slow speed to fly compared to the
graphical approach of just finding the tangent to the line.

Plugging in zero for the sink rate in equation V gives the same result
as plotting the tangent on the polar, I am not sure what equation VI
is finding.

Any one have some insight?

Thanks,

Tim