It looks like they mean (a) and (b) to represent the lengths of the segments with those labels. Also, they use left-to-right operation order rather than the tr

It looks like they mean (a) and (b) to represent the lengths of the segments with those labels. Also, they use left-to-right operation order rather than the traditional order. Less ambiguously (and correcting an error), (a) / (b) = ((a) + (b)) / (a)