The R in the formula consist of any organic group. As long the compound of
formic acid contains the carbon double bonded to an oxygen (carbonyl group) and single bonded to a the hydroxyl group, it
is considered to be a carboxylic acid. The hydrogen exists in this case
in the place of R in the same way that "R may also be H"(p.71) for the
molecule aldehyde. I hope this clears up some confusion.
Dan Casmier
casmiedm@plu.edu