In how many ways can 9 books be arranged on a shelf if 2 of them must be kept together?
Put one of the two particular books to one side.
Then there are
So there are:
#8! = 8 xx 7 xx 6 xx...xx 1 = 40320#
ways to arrange the
Then you can place the reserved book on either side of its companion. So for each of the possible
So the number of acceptable arrangements of the
#8!*2 = 80640#