In the last part of the question, we found
an equation for f, the fraction of the rope
that is hanging, and theta, the angle that
the slope makes to the horizontal. We used
this equation to find df by d theta and 
setting this to zero found the angle that
gave the largest possible fraction of the
rope not touching the platform. Now we provide
the theta that maximises f we can
just substitute this back in to our equation
and re-arrange for f.
